That doesn’t give some random stranger the right to track me down on social media though.
The point I made in response to the OP’s update was that it’s one thing to message someone on ancestry, because the assumption is that if you’re on there then you’re open to finding relatives. But if someone isn’t on ancestry themselves, then contacting them on social media is wrong, even if some member of the family has added their details to their own ancestry account.
In the OP’s case the apparent half sibling contacted her mother first. A woman who was a completely innocent and oblivious party in all this, and dropped a bombshell that she was fathered by her (the mother’s) husband 60 years ago.
That is good enough reason to block IMO. Just because someone was adopted doesn’t make them a nice person. And this person clearly isn’t one as they don’t give a shit who they trample on to get what they want.
They’d get short shrift from me, and it needs to be made clear that tracking people down on social media isn’t acceptable. It should be like an unwritten code. If you’re on ancestry then expect contact. If you’re not, then you’re probably not interested.
Respect is a two way thing, and as this person clearly has none, I wouldn’t have any compunction in blocking them.