I'm confused about my Ancestry DNA test

Question

On my DNA results page, initially there is woman who matches as Close family - first cousin so I assumed she could be the daughter of my great uncle (my father's uncle). Then a recent update occured and all of a sudden she is now half-sister or aunt. The amount of cms is 1734 we share 25% dna. The relationship never changed until recently and it's confusing on both ends. I don't know what to do. I don't know if she is my 1st cousin or my half sibling. I know she's not my aunt. I called Ancestry and the rep says well it depends on people who submitted their DNA tests and how they extracted their DNA and that changed the relationships on my results page. Another rep told me it's the system and it updates once or twice a year.

Answer 1

There are two things that Ancestry tells you.

The first is the amount of DNA that you and your match shares. This is measured in cM, and, as far as I am aware, does not change.

the second thing is Ancestry's suggestion (important word) of what the relationship between you and your match is.

They don't know you; they don't know your match. They have a large database of "other people's family trees", and they use them to estimate how you and your match are related.

If the other trees change, their estimate changes.

There are other tools, like the SharedCM tool at DNA painter, which can tell you what the possible relationships between two people who share 1734 cM are.

If you look there, you'll see about six possible relationships. You can apply your own specific knowledge of your family to determine which of those relationships is the correct one (or at least to eliminate some of them).