Half siblings using cMs (centimorgans)

Question

I used ancestry to obtain my results. I found my 1st cousins and aunt on my father's side. The process of elimination resulted that this one young lady has to be my sister but she came up my 1st cousin. There is no other men to test.

Looking at the range given for 1st cousins she is outside the perimeters.

Our shared cMs is 1170 which is outside the perimeters of 1st cousins but not quite in the perimeters of half-siblings.

1st cousin amount of cMs is between 680-1150 Halfsibling amount of cMs is between 1450-2050

What about the amounts outside the range?

Is she likely to be my sister?

Answer 1

The centiMorgan amounts you mention for the relationships are not hard limits but rather are ranges that such a relationship typically results in. Since inheritance is a random process, there will some cases where the inherited amounts fall somewhat outside the expected typical range.

There is a tool at https://dnapainter.com/tools/sharedcmv4 which estimates the likelihood of each possible relationship given the shared cM value seen. Try putting your 1170 cM sharing value into that tool, and you will see two sets of possible relationships. The probability of one set is about 91%, the other set has the remaining 9% of possibility. The 91% set includes the first cousin relationship (as well as others, such as half-niece). The 9% set includes the half-sibling and other relationships. So you two being cousins is much more likely than you being half-siblings, but neither is excluded.