What relation is my Half-Aunt to her DNA Matches?

Question

I have recently discovered through an Ancestry DNA test + GEDMatch that my Aunt is my half-Aunt and that she and my mother don't share the same biological father. Looking at my Aunt's DNA Matches on GEDMatch she has 2 close relatives who we don't know and who aren't a DNA Match for my mother or I. Jacob who shares 503.7 cMs with my Aunt and Jacobs's full biological sister Sophia who shares 347.8 cMs with my Aunt and is also an X-Match with her with total segments: 69.2 cM (36.405 Pct). Jacob and Sophia's X-Match is Total segments = 86.1 cM (45.349 Pct)

I can narrow it down to definitely being on their father's side as they have 4 half-siblings (shared mother) who my Aunt's not a match with. Also, Their paternal grandmother's side has Irish roots which explain my Aunts 42% Irish when my mother and I are 0%. Jacob is 36% Irish.

The relationship calculator says it's most likely Half-Cousin or 1st Cousin once removed but does the X-Match mean anything? I can't add that into the calculator.

Jacob and Sophia aren't their real names.

Update: I added my Aunt's DNA to MyHeritage and it came up that she has a DNA match with 179.7 cM shared. I contacted The match and we found a common ancestor that would make them 2nd cousins once removed (Which is exactly what MyHeritage DNA predicted) if my Aunt's father is Jacob and Sophia's great uncle. If my Aunt's Father is Jacob and Sophia's half Uncle that would make my Aunt's new match and her 3rd Cousins which the DNA calculator says is only a 9.88% chance. I asked the new match to upload their DNA to GEDMatch and they only share 41.1 cM with Jacob and 46.7 cM with Sophia. so I think this new information definitely rules out my aunt being their half-cousin. The DNA Jacob and Sophia share with the new Match makes it that there’s only a 10-14% chance they're 3rd Cousins which is confusing.

Well, The MyHeritage DNA Match has given me more information on her tree. I can't see the whole thing as it's on private because she's dealing with her own ancestry drama's. The information has raised more questions then it's answered. Her G-Grandmother is called Hannah, she thinks that Hannah's parents are her and my Aunts common Ancestors. But, I don't have them having a daughter called Hannah, I have them having a daughter called Anna born the same year. Which, they could be the same person, but Hannah married Anna's first cousin... I know what people think about small country villages but this would be the first 1st Cousin relationship or anywhere near that I've ever seen while using ancestry, it definitely wasn't as common as people think.

If Hannah was Anna and the MyHeritage DNA Match was my Aunt's 2nd Cousin Once Removed AND 3rd Cousin once removed wouldn't they share more then 179.7cM?

Answer 1

The advantage of more people testing is that you can use the cM ranges better. Here, your aunt matches full siblings with different cM totals, even though they have to be the same relationship to her.

You are looking for relationships where both 503 and 348 cM are within the range. Using the Shared CM Project, these include:

• Great grand aunt/uncle.
• Great aunt/uncle.
• Second cousin.
• First cousin once removed.
• First cousin twice removed.
• Half great aunt/uncle.
• Half first cousin.
• Half first cousin once removed.

I was able to rule out several relationships because only one of the siblings was within the cM range.

When a woman is an X match with another woman, it means nothing. It's like any other chromosome. But if a man is an X match with someone else, it means that portion of the match came from his mother.

The fact that Jacob does not have an X-match with your aunt, while his sister does, increases the chance that the match is through their father. But it does not at all rule out that the match is through their mother. Even though Sophia's X segment is large, there's a very good chance that Jacob would have no match there based on sheer luck. There's only a 50-50 chance that any given segment would be inherited by a given child (though it's not exact because segments break apart in unpredictable ways).

Since you already know that the match is through the siblings' father, the X-DNA is not telling you anything new.