How and Why to Research Your Collateral Ancestors
*This post may have affiliate links, which means I may receive commissions if you choose to purchase through links I provide (at no extra cost to you).
Have you ever thought to yourself that it’s not worth the time to research your ancestor’s brothers and sisters?
If so, you’re not alone. A lot of people feel this way, especially when starting to research their family tree. Why research sideways when you can keep building your tree up instead of out, right?
I’m Jessica, the professional genealogist behind Heritage Discovered. I help people move forward with their family history research when they’re stuck, overwhelmed, or short on time. If you’d like to hire help, my services can take the research off your plate while I track sources and results clearly.
Here’s what I’ve learned the hard way: collateral relatives often hold the exact clue your direct line won’t give you. A collateral descendant may preserve family details that did not make it into official records. When I build an extended family tree, I often find that the collateral family fills in the gaps.
If you’re having difficulties tracing an ancestor, investigating these extended family members can be the solution to breaking through to the next generation.
In this article, I’ll discuss what are collateral lines in genealogy, why they matter, and how to research them to further your family history research.
What Are Collateral Relatives in Genealogy?
Your first question may be - who is a collateral relative, anyway?
Direct relatives are those you descend from, like your parents, grandparents, and great-grandparents. They are also called lineal ancestors.
Collateral kin are the extended family of your direct ancestors. You’re not descended from them, but you share a common ancestor.
Collateral relatives examples are:
Brothers and sisters
Cousins
Aunts and uncles
Nieces and nephews
In-laws
Related posts:
Why You Need a Genealogy Research Log
Why You Should Search Collateral Relatives
It’s easy to get caught up in pursuing your direct line and pushing your tree back as many generations as possible. We all want to learn as much as we can.
Collateral relatives can give clues about those lineages when you hit roadblocks.
Researching sideways can not only help you identify clues to add more branches to your tree, it can bring to light family stories.
Questions like:
How large a family did your ancestor have?
Did any of their siblings die young?
Who attended school and for how long? Which siblings began working at an early age, and what type of work did they do?
Did any cousins, aunts, uncles, or other family live with your ancestor? If so, why?
Because they share a common ancestry, diving into these family members can give you important information about your ancestor that you may not be able to find in their records.
Related posts:
How to Strengthen Your Cemetery Research Skills to Find More Clues
5 Things to Look For in Probate Records to Help Trace Your Ancestors
How Collateral Relatives Help Break Brick Walls
Stuck on an ancestor? When I can’t move past someone, I dig into researching their siblings and other family members.
Researching collateral lines can help you solve your family tree mysteries in a number of ways.
1. Maiden Names
Finding the records for all of a couple’s children can help identify the mother’s maiden name. The same goes for researching the mother’s siblings. One of their records may hold the answer.
2. Birthplace
Sometimes it’s confusing (and honestly frustrating) trying to figure out where an ancestor was born. They might list different towns, states, or even countries on different records. Or they might not leave enough paperwork behind to sort out the truth. That’s where collateral relatives can help, because a sibling’s record might hold the detail your direct ancestor never shared.
And sometimes the records are maddeningly vague. Instead of a hometown, you get something like “Germany” or “Ireland” and nothing more.
I ran into this with an Irish branch of my own family. I could not find a specific place of origin anywhere. Then I found a tombstone for one of my ancestor’s brothers that named his birthplace: Gambonsfield, County Tipperary. Once I had that location, I was able to find a baptism record for my ancestor in that parish.
I never would’ve found it without researching the siblings. To this day, that tombstone is the only source I’ve seen that gives a specific birthplace for anyone in that family.
3. DNA Matches
One of the best resources for your genealogy toolkit is an Ancestry DNA test. If you know the surnames of your collateral lines, it can help you figure out how you’re related to your DNA matches.
This is especially useful for when your matches don't list family names (or only a few) on their profile. I’ve been able to identify how I’m likely related to someone, and on which branch just by looking at the surnames in their tree, even if the match has a very small tree or there are no common ancestors between us.
AncestryDNA gave you results, but not a plan. Learn what to click first, how to read matches, and how to turn shared DNA into clear next steps. Read more.
4. Name Changes
Lots of families changed their names after arriving in the US. If your family’s last name was Anglicized or otherwise altered, collateral family can pinpoint their original name.
5. Sorting Out People With the Same Name
If your ancestor has a common name or the family reused first names with every generation, collateral relatives can clear up who is who. This will ensure you’re researching the right line. (I’ve got 5 people named George Henry Burke in my tree!)
6. Pre-1850 Records
Before 1850 (the first census to list all household members), there are fewer records documenting family by name. Some disappeared over time and some were never created in the first place. Looking into all the collateral relatives increases your chances of moving back in your lineage.
Pro Tip: If you want to publish a family history, including collateral ancestors will make it more authoritative.
Related posts:
6 Common Genealogy Mistakes And How To Avoid Them
Why You Need to Use Libraries and Archives in Your Genealogy
7 Simple Steps To Creating A Genealogy Timeline (And Why You Need One)
How to Research Collateral Relatives
Collateral ancestor research is one of my favorite ways to get past roadblocks and learn more about a family. The trick is that collateral kin are not always obvious. Some are right there in the records, but you do not notice them yet.
The quick plan
I do this in three steps:
Spot extra names linked to my ancestor
Add those people to my working list
Research them like I would a direct ancestor
Step 1: Look for “extra names” on your ancestor’s records
Start by scanning the records you already have for any names that show up with your ancestor. Some will be clear family. Others might look unrelated at first. Either way, write them down.
Focus on:
Witnesses (marriages, baptisms, burials)
Neighbors (especially on censuses)
People in the same household (boarders can still be family)
Anyone connected to land, estates, or legal paperwork
Step 2: Use records that naturally reveal family ties
These record types often name relatives, hint at maiden names, or point to a birthplace. They also help you build a stronger extended family tree.
Vital and church records
Birth records
Marriage records
Death records
Census and community records
Censuses (Don’t forget to look at who lived next door to them!)
City directories (same address, same surname, repeat households)
Legal, land, and money records
Probate records (heirs, executors, bequests)
Deeds (family transfers, shared land, witnesses)
Pension records (affidavits and support statements)
Newspapers and migration records
Social Security applications (SS-5 forms)
Step 3: Track what you find so you do not lose the thread
Many of these sources include key hints like birthplaces, maiden names, and family relationships. I like using a simple genealogy records workbook or tracker so I know what I have and what I still need to search.
Step 4: Research collateral relatives like direct ancestors
Once you have a short list, treat each person like they matter, because they do.
For each collateral relative, ask:
Where were they from?
What was their maiden name (or their mother’s maiden name)?
Did they migrate with my ancestor?
Do they show up as witnesses, neighbors, or informants?
Do their records name the parents I am trying to prove?
This sideways research is often what helps me break a brick wall, confirm a wife’s maiden name, or finally narrow down a hometown.
Related posts:
How Setting Genealogy Goals Makes You A Better Researcher
7 Tips To Help You Create A Successful Genealogy Research Plan
How To Build The Best File Folder System For Your Genealogy Papers
How Collateral Lines Have Helped My Genealogy Work
Besides the example I shared earlier about my Irish ancestor’s birthplace showing up on his brother’s gravestone, collateral research has also helped me tackle one of the biggest brick walls in my family tree.
The roadblock is a couple named Samuel Jones and Sarah Merrill. Both names are very common. To make it harder, they had nine children with common first names, too.
After years of searching, I finally got a Civil War pension file for their son, Henry.
Henry died in the war. To claim a dependent’s pension, Samuel had to prove he was Henry’s father. Sarah had already died, so the file could not rely on her testimony. Instead, the claim included affidavits from two of Sarah’s cousins.
Those affidavits were my first real clues about the Merrill family.
From there, I started researching the cousins and building their lines out. That collateral work has very likely led me to Sarah’s parents. I never would’ve found that lead if I had not researched all nine children and tracked down every record I could.
If you want to hear more about this story, I shared a two-part post all about it. Read: Part 1 and Part 2.
Helpful Tools for Researching Collateral Relatives
When I’m building an extended family tree, I like using simple worksheets so I don’t lose clues as I research collateral relatives and their households. These shop items pair well with collateral research:
Genealogy Records Workbook: Helps me track which record types I’ve checked for each person in the collateral family, and what I still need to search.
Locality Research Navigator: Keeps me focused on what records exist for a place before I spend hours searching the wrong thing (or missing something in an archive).
Brick Wall Breakthrough Blueprint: Helps me sort clues from collateral relatives when the evidence conflicts.
Final Thoughts on Collateral Relatives
Sometimes it takes a roundabout route to track down your ancestors and add to your family tree. But by taking the time to research all of your direct ancestors’ siblings and other extended family, you’ll increase your chances of finding the answers you’re looking for!
If you want help identifying which collateral relatives to research, or you’d like someone to track down records from libraries and archives for your collateral family, you can browse my services and reach out. I’m happy to help you take the next step.