Half sisters or aunt/niece relationship?

Question

I do not know who my father is and a woman (who also doesn't know who her father is) recently came up matching me on Ancestry.com as a close relative. She and I both uploaded our raw data to gedmatch.com.

For the Xdna we matched:

Largest segment = 196.1 cM Total of segments > 7 cM = 196.1 cM Actual.

For the autosomal DNA we matched:

Largest segment = 137.3 cM Total of segments > 7 cM = 1,786.1 cM Estimated number of generations to MRCA = 1.5

641785 SNPs used for this comparison.

We've narrowed this connection down to my father's side because I match an X on people who are related to each other, but who do not match my maternal side at all. My question is, does this indicate a half sister or could this still be an aunt? She's too young to be a grandparent.

Thanks in advance for any light with more experience with this can shed on this situation.

Answer 1

You are right to suspect an Aunt/Niece or Half-Sibling relationship based on 1786.1 cM Estimated Autosomal DNA matched.

You haven't specified if you know who your mother is too, but if you do, or for the benefit of others that might be in the same situation. I know for a lot of people in this situation it could be quite a sore subject depending on circumstances, but find out as much information on your father from your mother as possible: Names, Acquaintances, Known locations, Jobs, anything that you could potentially match up with any details your match can come up with. This could narrow down your search to certain locations or groups of people.

If you don't know your mother either, then depending on where you live and your age there could be adoption records that might be relevant to you.

Another thing you could try is trying to go from the top down on trees that the people you both match on, i.e going forward in time rather than back. This could potentially narrow lines down if they go to locations based on any information you might know about your father.