Unsure about DNA result,is this lady really my aunt?

[Tacsi]

Hi,
I did Ancestry DNA a while ago and although I don't fully understand CM ect...I've been really confused about one of my relatives.

I've always known about this relative,don't know her too well but have always known her as my aunt. My mum's half sister(same mother, different father's). This lady didn't grow up with her mum(my gran) or any of her siblings,but with extended family.

Our match has come back at 270CM and is showing we are 2nd-3rd cousins. No one in the family can explain this and all believe she is my Aunt. I feel different but don't want to rock the boat so to speak,so have left it. But for my own research and piece of mind I'd like to know. I've other cousins that I know 100% are cousins with higher CMs of 400+

We have lots of matches in common from my grans side, but we have no matches at all in common through my grandads side(I have a lot of matches to my grandad on ancestry).

Does anyone know if it's at all possible that this lady could be an Aunt/my mum's half sister with a CM of 270 or are Ancestry correct in that she is is a 2-3rd cousin?
Thanks

Because she’s only your mums half sister she wouldn’t show as an aunt. It is t the same degree of closeness as if they were full sisters

Edit: I meant we share great gran matches but no great grandad matches at all ,which also doesn't make sense

Think I need a diagram

Thanks for reply.
I did expect a lower cm with "aunt" being half sister,but I've other 2nd-3rd cousins that I know are definitely cousins with higher CM. As an aunt we apparently share my mum,gran and my great grandparents ect... as relatives, I thought (rightly or wrongly) we'd have a much higher cm 🤔

@Tacsi

Have a look at this link which shows levels can vary quite a lot and gives half relative levels too:

fh.familysearch.org/system/files/team/ait/images/blog/centimorgan-chart.png

According to that list even a half aunt would be over 500 CM.

For comparison OP my half first cousin shares 377cm with me and a bit more with my brother .

So I think a half aunt is unlikely.

Half first cousin is 137-856 CM on the list!

For your mum's half sister I would expect to see a higher CM score. Maybe double what you have. Could it be possible that this aunt was actually the child of your grandmother's sibling, but referred to as your grandmother's child to protect the birth mother's dignity/reputation?

As in, could she actually be your mother's cousin rather than her half-sister?

That does suggest that perhaps there was another member of the family whose child was given a title of 'Grandma's' or a different father adding to a DNA distance.

It can get very complicated with families the further you go back.

I am thinking along the same lines as AnotherSpare and NeverDropYourMooncup

Thank you for all the replies.
I've suspected for a while that she isn't an aunt because of her "mother's" (my gran) parents who are my Great Grandparents and would be her grandparents, we only share matches through her gran/my great gran and no matches at all through her grandad/my great grandad, which absolutely doesn't make sense if she was my mum's half sister.
I have a strong theory of where she fits in, in the family but definitely can't say/ask as I would never want to upset anyone. It's just for my research,peace of mind ect...

Replies are making sense and giving me more food for thought, thank you

DNA painter suggests these relationships. I agree with the suggestions the others have made as to what may have happened.

Does your mum have a sister who would be of child-bearing age (say 14 or older) when your 'aunt' was born? Is it possible that your 'aunt' is really your mum's niece but brought up as a daughter of the house rather than a granddaughter?

Alternatively is she actually your mum's cousin and your gran had a sister who either wasn't married and wasn't allowed to keep the baby, or couldn't cope with her and so your gran took her in and raised her as her own child? That would make her your 2nd cousin...

I'm pretty sure ,but obviously not certain my grans brother Is 'aunts" father, which would make aunt my mum's half 1st cousin his mum(my great gran) brought her up but everyone was told she was my grans.
My gran left home really young and was forever known as "that awful woman" who abandoned her child, she still is known as that to a lot of family members even though she passed away a very long time ago. It's really sad as I'm pretty sure aunt wasn't even her child to "abandon"

Does aunt know who she is?
You now have the dilemma of whether to let her know that story everyone told to her, is wrong.

I've only been back in contact with Aunt for past few years, from everything she says, she thinks/sees my gran as her mum. Part of me from other things she has said thinks she may know or at least suspects but has made it obvious she doesn't want to go digging up the past.

I still call her Aunt and we do get on well,she is lovely. I've chosen not to say anything as I absolutely know it will hugely upset not just her but other family too. I have broached it with closer/more understanding family members and they don't think I should say/do anything either as the fallout will be huge

Have you looked in BMD indexes to see if there are any clues there? Or looked on parish records for a baptism?

Yes, can't find anything relevant to aunt only grans information

And actually depending on her age, you might be able to order her birth certificate. If she's under 50 you'd have to provide details you don't have, but if she's over 50 all you need is what you can find on the BMD index. Obviously would be up to you if you chose to do that, and if you uncovered something whether you chose to tell her. Presumably she has her own birth certificate so knows what it says.

So if you can't find her birth, that then leads to the conclusion there's something wrong somewhere:
a) you have the wrong DOB so searching the wrong indexes
b) she wasn't born in the area you think she was so you haven't found her birthcl
c) she was registered under a different name (either forename or surname)
d) the index is wrong

If you are correct and grans brother is the father, she may have been registered with the mother's maiden name, which fits with your theory.