I have been using Ancestry.com for years. A year back I submitted my DNA test. I have had quite a few matches that land in the distant relative to 3rd or 2nd cousin range.

Recently, I was notified of a ‘Close Family to 1st Cousin’ match. We match 1705 centimorgans over 52 segments.

From my novice level of understanding, this suggests that this match could be: a grandparent (ruled out - based on age), a niece (ruled out because.. duh, I’d know) or half sibling (hmmm..).

The match and I have spoken. She is adopted and knows nothing about her biological parents except for some ancillary bits of information that was given to her by whatever agency she worked with in an attempt to find out about her biological parents.

Interestingly, there are a few bits of ‘story’ that seem to match to my (deceased) Father:

My ‘match’ was told that her biological father was born between 1922 and 1932 (my father was born in 1927)

My ‘match’ was told that her biological father had 7 siblings (my father had 6 siblings - including him, equaled 7)

My ‘match’ was told that her biological father lost a sibling in their youth due to an accident with a baseball bat (my fathers twin brother died at 9 from being hit in the head with a swing... or the story goes..)

My ‘match’ was told that her biological father was deferred from the army due to having Rheumatic Fever as a child (my father was the only boy out of his 7 brothers to have Rheumatic Fever and was barred from service)

Finally, My ‘match’ was born in Milwaukee, Wi. (my father was born and lived his life in Appleton, Wi. However would frequently travel to Milwaukee in his 20’s)

There was ancillary information about my matches biological mother, but none of that seemed to correlate to anything I knew.

Have I found a long lost half-sister?

  • Yes, it is very probable that she is your half-sister. But it will be great to get more tests from relatives to make stronger conclusions Commented Feb 12, 2018 at 17:44
  • So if my 2 full siblings also take the DNA test, the comparison to my ‘match’ will further support half-sibling? Wouldn’t I expect the same results as mine?
    – WSchroeder
    Commented Feb 12, 2018 at 19:34
  • If you get the tests from your siblings, nephews/nieces, uncles/aunts, it is possible that you will be able to reconstruct the DNA of your parents. And then compare these kits against your 'half-sibling'. Also there will be a little skew between your and your siblings tests against this 'half-sibling' because every child gets from particular parent 50% of DNA, but these DNA differs between childs. So one child may look totally different than another.... more data leads to more precision and more confidence Commented Feb 13, 2018 at 14:03
  • I'd recommend the service called GEDMATCH for such an investigation because it provides wide set of tools Commented Feb 13, 2018 at 14:04

3 Answers 3


Yes it is very likely you have found a half-sibling.

Although your story is different, this question is almost the same as Distinguishing half sibling, uncle or half nephew using Ancestry DNA?

See my answer there, because at 1705 cM, a Cluster #3 relative is still a slight possibility for you, and that includes 1st cousin. But you will likely need your father's side to be related to your mother's side for that to happen.

  • Is there a test that can further determine the actual relationship?
    – WSchroeder
    Commented Feb 12, 2018 at 15:01
  • Unless I'm reading it incorrectly the range suggested by the Aug 17 version of the Shared cm project is 553 to 1225 for a 1st cousin. The cm range for a half-sibling is 1317-2312. Schroeder's 1705 match accompanied with the ancillary ( & oral) info, there is a extremely strong case for the half-sibling relationship.
    – BobE
    Commented Feb 12, 2018 at 15:05
  • @BobE - I was using Cluster #3 from Table 1, The Cluster Chart which groups all relationships that have identical expected cM together. It gives a 99th percentile of 1761 cM for the cluster that includes 1C. But you're right that for 1C, the range in Figure 1, The Relationship Chart, only gives a 99% of 1225 cM. Now that you've got me to look at this more closely, I'm guessing that cluster 3's 99% results are inflated because of the range for Great-Aunt/Uncle/Niece/Nephew which is in that cluster and has a 99% percentile of 2108. Interesting. I'll have to re-think. Thanks.
    – lkessler
    Commented Feb 12, 2018 at 18:14
  • @BobE - With regards to "the ancillary (& oral) info", if his "match" was a child of his father's brother, then most of the info stated would have applied as well. I agree with you though that the Rheumatic fever supports the "match" being a half-sibling.
    – lkessler
    Commented Feb 12, 2018 at 18:26
  • 1
    In case you missed it: Results for 1705 cMs from the interactive version of the shared cM project at DNAPainter: dnapainter.com/tools/sharedcmv4/1705
    – Jan Murphy
    Commented Feb 12, 2018 at 22:55

So I recently used a confidential intermediary, which is someone who can legally access closed adoption records. To my surprise, I found I had not only one brother, who I had been searching for, but six other siblings. One of my siblings took a ancestry DNA test as I had and results came back with a close family to first cousin connection.

Another mystery remains for us as to whether we share the same mother or the same father, as our DNA matches are not exactly what they should be if we share the same mother but they are what they should be if we share the same father. And my aunt could possibly be her mother...crazy story I know, but she is your half-sibling.

I downloaded a DNA cM tool chart which breaks down every possible relationship based off of the cM number. When you spend a lifetime or even a partial amount of your lifetime looking for something or someone and get answers, run with it. Don't question it. She is definitely for sure your half-sibling, just as my beautiful sister is my half sibling, although we call each other just sisters.


It sounds like a half sibling. Typically a half sibling is approx. 1750 centimorgans. Full siblings are 2500+

  • 2
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    – Jan Murphy
    Commented Aug 7, 2019 at 19:15

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