This is a follow up to Probability of Specific Relationship from AncestryDNA?

I have an Ancestry match with two half-sisters (same mother, different fathers). The older one by 16 years matches me at 684 CMs and the younger at 461 CMs.

Using the DNADetectives chart, these suggest two possibly different relationships with me although there is a very narrow overlap.

How could two sisters with a common mother, match to me with such a gap in CMs?

Yes, knowing your relationship to them is key - it's possible, for example that you're related to one of them through both parents and the other through only one.

Uncertainties like these are one of the reasons why DNA is an adjunct to, not a replacement for, traditional genealogical research.

  • Thanks for the replies. I should have been clearer. They two of them are half-sisters, same mother different fathers. Neither of their fathers is related to me. We have been doing the traditional genealogical research but as yet have nothing definitive - which may partially be the result of their mother having been adopted at birth. My Ancestry match with each of them is different as noted. My question is about the difference in match we me that each had. Thanks GEORG GAAL, PolyGeo, and Cleaverkin. – TommyJoeK Oct 17 '17 at 23:42
  • @TommyJoeK believe me, this difference is not impossible, but VERY possible. If you'd check the chart from DNADetective, you will see that there is great distribution in amount of shared DNA between one person and it's 1st cousins (or 2nd cousins, or ANY non-direct relative, i.e. not parents/child or sibling) – GEORG GAAL Oct 18 '17 at 6:58

Please clarify the question. It is totally unclear whether they are YOUR half-sisters. Also it is very strange because of the small overlap of DNA between you and these half-sisters.

Regarding the possibility that they are 1st (or more distant) cousins to you and half-sisters between each other, the provided numbers are very possible.

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