How to resolve this?

[Hairyhat]

Not aibu really but I hope it's still an interesting enough subject for everyone to read. I need the footfall for advice for a friend.
She has a half sister that she used to be close to (while they were children) but the sister didn't know they were related. The duplicity of living the lie made my friend distance herself slightly from the friendship (during their teens) and other everyday growing apart and different interests led them to not be so close for a number of years. But still they remained in touch and fond of each other. As friends.
The reason for my friend NOT telling her sister they were related was that her mum had asked her to keep the secret in order not to hurt the other Mum's feelings. (I hope this makes sense). I think the mother of my friend was for a short time the OW, but nothing developed and perhaps it was a ONS or similar, resulting in the birth of my friend.
The marriage remained strong with no other affairs for over 50 years. As far as we know.
The dilemma my friend faces is this: now BOTH mums have died, should she tell her (as far as I can gather, lovely) half sister that they're related? Have the same dad?
She spoke to her (absent) father to ask if she could now say they were sisters (after the death of his wife) and he said no, it would hurt her (the daughter) too much. At the time my friend decided to leave well enough alone, mainly because her half sister had just lost her mum but also out of respect. This was a few years ago.
The dilemma is, if she waits until their father dies, (he's 89 but in good health) then this lovely innocent sister has no way of confronting or talking to her dad to confirm things or get closure on an affair from 50 years ago. But at the same time, my friend would love to deepen the relationship she has with her only sibling and to come clean as it were.
My advice is to tell the lovely sister. She has no loyalties to this absent father and no one else can be hurt. Life is too short and their children can become cousins!
But what do you MNs advise?

I’d tell her, but be prepared for a backlash of emotional overload and the friend taking it out on her half sister.

She knew all along - how will the sister take to that news?

I’d tread carefully

I would tell her definitely

I would tell her and hope that she can see the complexities of the promise your friend made to her own mother in order to spare the feelings of others.

Surely at the end of the day the relationship is something to be rejoiced at; finding living relatives when you may believe you have none has to be welcome news. But it may leave an unnecessary scar if the biological father dies and there's no opportunity to talk to him or otherwise resolve any issues.

She knew all along - how will the sister take to that news?

Yes, this would be my biggest concern.

If I was the sister who had been lied to I'd be most upset with my so-called friend than anyone else. Don't get me wrong, I'd be absolutely beyond livid with my parents for lying to me my entire life, but I would feel a huge sense of betrayal from someone who had professed to be my friend all these years whilst knowing and withholding something so fundamental about me (ie. my parentage).

When it comes to infidelity, I believe the feelings of the adults concerned should come very secondary to the feelings and rights of the child (and yes, that unfortunately includes the cheated on partner). Unfortunately, that ship has long sailed in this scenario, so the question remains of whether to tell the unsuspecting friend the truth.

The friend certainly has a right to know. By rights, her coward of a father should be the one to tell her, but obviously won't. His feelings are irrelevant here and if the friend is to be told then it should be before he dies, so she can confront him if she wishes to.

I probably would tell her, but be prepared to lose the friendship, and/or be in a 'shoot the messenger' type situation.

Is there any chance at all that the sister already knows or suspects that she is your friend's half-sibling?