My sister did ask my mother last year when it first came to light and she muttered something about "what good is it bringing it up now?" and changed the subject so she knows something
Will you let your mother keep her secrets ? It might be something she's not proud of i.e. an affair, sexual assault etc (twice with the same person, yep, happens all the time.)
Your Dad might not have known that his kids weren't, 'his blood.' Your Mum might not want you to know that she lied to your Dad about his own kids.
If it turns out your Dad is actually your uncle, then, what next ?
Does your Mum have to spend the remaining years of her life feeling guilty and ashamed all over again ? Does she have to explain it to her own now adult kids ? Right to know ? Really ?
Does your Mum really have to admit she fraudulently said your Dad was your father and had no problem with it being recorded on your and your sibling/s birth certificates ?
How much heart breaking infertility and possible miscarriages did your Mum/your parents have to put up with. What sort of, 'private arrangement' did your Mum end up with because she was desperate to start a family ?
Checklist, have you seen:
Your parents' birth certificates
Your parents' marriage certificate, marital status of both parties as recorded there.
Your birth certificate, who is recorded as the father ?
Your sibling/s birth certificate, who is recorded as the father ?
Grand parents' marriage certificates which of course will record marital status of parties that got married.
Sorry to say this but don't be a stooge for your sister then have to live with the emotional fall out from your mother.
Do you want your birth certificate corrected ? Do you want a blank space where your Dad's name used to be ?
Upsetting, isn't it ?