You ask people not to analyse the situation, but for me that's actually one of the ways to cope with it. To put that betrayal into context, what either of you were hoping to achieve or gain, your family dynamics, whatever the heck your sister might be up to. Not assigning blame or responsibility, on the contrary, taking painstaking effort to stick to just seeing causes and effects. My own experience is that such understanding (even if partial) can go a long way towards bridging the gap between still wanting to love the person for all the good parts, but also feeling like I should disown them for the bad. Or myself. It kind of helped with the big rupture with xT. Only kind of, because we kept getting stuck in the 'blame game' (but then she got very defensive very easily, and so do I) and because all the logic and rational thinking in the world can't seem to replace emotional validation (which, almost felt like xT was making a point of not giving me? Well, in any case, I wasn't getting it).

Otherwise, seems like you're already doing everything I could think of.

Also, umm sorry, analysing again, but it feels important ... what really hit me was trying to imagine your family situation ... what turns this thing from a confused 'right, technically a breach of confidence, but no indication that anyone found out anything they didn't already know' to 'o **** this is bad' in my mind is that somehow by not acting the way you agreed on (well yes that's already problematic in itself), and you told your family you expect her to, she did give out some difficult-to-grasp but very visceral bit of information about the connection between you, and who knows what else she might have revealed implicitly and unwittingly, even if she was very careful not to divulge anything and made sure the card was as generic as possible. This sort of stuff could be completely harmless with a trustworthy and supportive family, but could cause a lot of hurt with a suitably toxic one. And in any case, it really isn't anyone else's business.