@sozobe,
Anyway, this reminds me of a situation with another branch of my family. There are two sisters who have different fathers, but they don't know that.
Their mom met her current husband when sister #1 was still a baby, and sister #2 came shortly thereafter. #1 looks very different from the rest of the family (a lot like her biodad, who she doesn't know exists).
We don't see this branch often, and somehow this had come up with sozlet and she knew about it. Then it was time to see them, and she was charged with not telling anyone (including other kids she was hanging out with, who could then tell the people involved). It was an uncomfortable conversation since we're generally anti-secrecy. She got it, and didn't say anything, but it was stressful to be in that situation. (As it happens, I found out when we were there that #1 probably does know, but since officially nobody knows anything, she hasn't spoken to #2 about it, who probably doesn't know, though she may just be going along with the family's preference and does know but pretends she doesn't... argh!)
I'm sure it's stressful to have that knowledge thrust upon you in such a flukey way.