I recently did Ancesty DNA to determine a match between me and a possible half sister. The results came back, and we share 1366 cM of DNA. As I’ve learned, this is on the low side for half siblings and the very high side for first cousins. Is there a way I can determine which is correct given that the two potential fathers are deceased? Based on the family story, it is more likely that we are half siblings but the low number leaves lingering thoughts that my uncle could have been the father. I understand that it would be rare that we are first cousins with that number but isn’t it also rare to have such a low number for a half sibling?

I think a higher number is more likely in general through cousin marriages.

To get a much lower number it requires some “bad” luck on the randomness of DNA but it usually is a couple generations.

What I’d suggest is that you look for other matches and how they related to everyone. The more data points you have, the clearer it becomes which hypothesis is correct.

You can plug your shared cM number into the DNA Painter tool here:

https://dnapainter.com/tools/sharedcmv3

The tool indicates that 1366 cM is beyond the 99th percentile range for 1st cousins. The only same-generation relationship candidate is half-siblings.

I would consider Blaine Bettinger a reliable source on this.

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