Is this Match a Half Niece or a Second Cousin?

Question

A DNA match for a woman has shown up on my Ancestry DNA as a second cousin with 490 cM. I recently discovered that I am thought to be the half sister of this woman's mother. My full sister shares 446 cM with the woman and my full brother shares 363 cM with her.

Could this girl be the daughter of a half sister?

Using https://dnapainter.com/tools/sharedcmv4/490 calculator I entered each of our cM values that my full siblings and I have in common with the woman.

The generated chart showed a 4.47% chance that I could be the woman's "half" aunt. I understand it to show a better chance of me being a second cousin. I also checked my sister's at 446 cM and it only showed a 1.5% chance. My brother's at 363 cM didn't show at all as a "half" uncle.

I'm really confused by all this. From what I have read, these numbers fall below the minimum of the range. My father also has 4 brothers. My father and his siblings are all deceased. My sister and I have uploaded our DNA to GEDmatch. We are hoping the woman will do the same.

Should we be able to determine the relationship through GEDmatch?

So far, Ancestry has been very accurate in predicting relationships.

Answer 1

These numbers are too small for this to be your half niece.

You need a relationship that works not just for your match but for the match each of your siblings has. This means it has to be possible for any match between 363-490 cM. The range for a half niece would be 500-1446 cM (using Blaine Bettinger's work). While I wouldn't dismiss 490 as being not quite 500, the results from your siblings pretty much rule this relationship out.

This fits well into half great niece territory though. Range 125-765cM. You could be the half siblings of this woman's grandparent.

Other good potential relationships are:
Half first cousin (137-856cM)
Half first cousin once removed (57-530)
First cousin once removed (141-851)
Second cousin (46-515)

Gedmatch is an excellent site and I highly recommend that everyone upload their raw DNA there. You'll be able to look at the actual segments that match and this will give you the ability to triangulate with other relatives that might have tested. But it won't give you a better prediction to relationship than you have now.

Your only ways to solve this mystery are to test more people who can narrow down the options or to work the paper trail. Of course, your best option is to do both. Good luck!

Answer 2

You don't mention the relative ages of the principals, but if this woman is much younger than you, half-great-aunt is another possibility indicated by the Shared cM numbers, which I think would mean that the other woman's grandparent (not her mother) might be your half-sibling.

There's another tool on the DNA Painter site that you might want to look into - the probability ("What Are the Odds?") trees:

https://dnapainter.com/tools/probability/

You can either build the tree with what you know of the other family, then create hypotheticals for yourself, or (if you have access to one of their samples), build a tree of your family, add their matches to you and add hypotheticals for them.

The DNA Geek postings recommend that a given hypothesis have a 10X greater probability than the next-best to reach a statistical certainty.

I've used these trees successfully for identifying a number of things - an NPE for someone's grandparent, half-sibling relationships, and proving previously-undiscovered descendant lines. I can't say enough good things about this tool, it's the best thing I've seen in years (I have no affiliation with DNA Painter, DNA Geek, or anyone else).

One of the things they're particularly good at (given not very many samples) is identifying relative generations. It turns out that grandparent-grandchild, aunt/uncle-niece/nephew and half-sibling all look very similar in shared cM, but with a few additional cousin samples, it's much easier to pinpoint relative generations.

Answer 3

It is most likely that the woman is the granddaughter of one of your father's brothers.

1. The numbers match ("First cousin once removed 141-851cM").
2. The ages match.
3. There are several ways the woman could be confused about which brother is her grandfather.

Of course, more testing could settle the matter.