Friends betrayal

So you expected a teenager who is now an adult to deal with it in an adult to behave in an adult way?!
YABU in the way you feel, but "think before you jump"- so to speak ...e.g. it wasn't her fault your mum had an affair. It was your mum's fault. It wasn't your friend's secret to tell.

A teen who us now an adult to deal with it an adult way.
Yes she's an adult now...but she was a child then .

She’s a bit dim for telling you she knew. It wasn’t her secret to tell but I don’t know why she had to let on she knew the whole time.

You're angry at the wrong person. Your parents could have told you.

She's dumb for admitting she knew though, given that she's the only one left for you to be angry with. Pointless.

It wasn’t her secret to tell though was it. I’m very impressed by her capacity to keep a secret.

You’re transferring anger from your mum to your friend.

It wasn’t her secret to tell, she hasn’t done anything wrong. I wouldn’t have told you that I knew though, for exactly the reason you’ve reacted like this.

I have a similar situation. My brother (much older) found me. My parents are both dead as well. I get the secret being kept while my mum was alive and possibly even my dad (brother is not his child). So many relatives have since told me how happy they are that I am not alone any more. It has destroyed my relationships with them all - I can’t trust them any more. I feel like there is more they aren’t telling me.

so I completely get that your friend was in an impossible situation - and didn’t do anything wrong - but I also get how it feels like a betrayal.

You have just found out that your friend is immensely loyal and trustworthy.
It was not her fault that an adult told her a secret and it was not her secret to tell.
She kept the secret like any person should.
And how could she know if it were true or false?

Forgive your friend.
Remain friends.
Have you forgiven your mother?

Ask your new sister to have a DNA test and lodge it with Ancestry or similar.

It’s not her fault and she probably didn’t even think about it if she found out in her teens. It’s important in your life not hers. When asked she has told you. Isn’t that good enough?

100%.
As an adopted person I feel for the friend completely agree that the friend would have been thrust into an impossible situation.

How would your friend have known that it was a big family secret being kept from you? Surely, she knew and assumed you knew, but perhaps also felt she shouldn't know or mention it as it was private and nothing to do with her.

PS
How lovely discover a new sister at this time in your life. Cherish her!

@Aluna "Friend chose to be complicit in OP’s mother’s betrayal."

What an awful thing to say. The friend was a child when she was told. Whoever told her was very wrong to share such adult information with her and even more so by swearing her to secrecy. To call a child complicit in the sexual affair of her friend's mother is unbelievable.

OP dont blame your friend. She was put in an impossible position. She could have lied and pretended she knew nothing. She told you the trurh eventually.

@andromeburns you've no need to answer this - it's your life that's had a seismic shock and no one has the right to push you - but if you care to say, how are you feeling now? there must have been some thoughtprovoking points in all the posts here.

I still think you need to put this behind you and focus on the positives. Society is much more operand less deferential to authority than it was. It would've been much harder to breach confidence then than it is now.

I think your betrayal is misplaced. This wasn’t your friend’s information to share, you were not entitled to know about this just because she did.

Your mother gave birth to your sibling and pursued adoption (whether it was her choice or by force). That must have been horribly traumatic for your mother, to lose her child. If she had wanted you to know about this she would have told you herself. It was not your friend’s place or responsibility to fill you in, either before or after your mother’s death.

Yes .... there's a lot to come to terms with at the moment.

While I completely and totally get why you're feeling betrayed, not sure I could let it go myself, I'm glad that you will stay remain friends.

I knew some family secrets as a small child and I also instinctively knew I was to not say anything . As I got older I felt it wasn’t my business to share . I also knew that I would be a family pariah .

You seem to have the wrong end of the stick. I’m not saying she’s complicit in the mother’s betrayal of her husband, I’m saying she’s complicit in the mother’s betrayal of her daughter by not telling her she had a sister.

I wouldn’t expect her to say anything as a teen, but as it became clear the mother intended to continue her deception into adulthood and beyond, that’s where the line should have been drawn.

So she's told you the truth without upsetting and destroying your family and you feel betrayed? She sounds like an amazing friend.

No she never told her the truth, when OP found out by other means she admitted she knew, that’s not the same thing at all. She has massively upset OP who feels betrayed. Even if the friendship survives, OP will probably never trust her again.