I think imagining that if the OP’s friend had spoken out earlier, everyone would have sat down and had a civilised conversation is pretty naive.
Goodness gracious me, I don't thnk that it would have been easy.
Blood relations are one of the things that people react to in the widest of possible varieties. Discovering you have half-siblings means nothing to some people. To others, it goes to the core of whom they are - and there's a wide range of reactions in between.
You can't, if you have any actual experience in this area at all, just say "it's nothing to do with the OP". Yes it is. It's her half- sister.
Knowing that you have a half sibling, knowing they exist, makes a huge difference to how you react when they contact you. The sheer shock of discovering your own parent had this giant secret is a slap in the face and saying "it's got nothing to do with her" really is naive. Then you have to actually come to terms with the fact you have a half-sibling. It's easier to cope with contact with a half-sib if you know they exist, whether you reach out or whether they do.
And there's a considerable chance now that the OP might have discovered she has a half-sib through DNA testing, as so many other people have. In some cases it's worked out, in others it's been devastating.
And fyi, I've held my own biological mother when she wept, heartbroken, at having given up her baby because she had to, pre-1975. It's not pretty. And it really, really can have an impact on the mother's other children. Perhaps not in all cases, but in very many, in fact probably the majority. Denying that - now that's naive.