My sister and our female cousin have joked for many years that they are actually sisters, my sister being my aunt’s illegitimate child (of a different father, of course), raised by our mother. Can DNA testing of those two alone determine this?
Wild guessing:
DNA testing between the two of them might be indicative but not conclusive: First cousins would probably have 12.5% of their typical marker genes in common, while half-siblings would have around a 25% match.
Being able to test your sister against either of your parents or your aunt would give much stronger results, of course, since that’s a standard parenthood test.
Ah, yes, I forgot to add, we are of the avant-garde generation. There are no living parents or aunts or uncles to test. I’m thinking that if I am tested, that might provide a point of comparison. My sister and I should have close to 50% coincidence, but our cousin should have 25% at most. Yes?
It should certainly help. I’ve done DNA testing for genealogy and part of the DNA data has something called cMs and longest block. Between the time, it’s able to calculate how closely someone is related. There would be a much higher relationship in the DNA numbers shown for a sister than a first cousin. There is a range for all these relationships, but it should put you confidently within a “bucket” of which each of you are.
What I recommend each of you do, it go to Family Tree DNA and for $99.00 you can get the Family Finder DNA test. This will do an autosomal match up to five generations. It will produce matches for you and how closely you are related as I mentioned.