Elena I relate totally. If anybody asked me right now why I wasn't working I would feel mortified (I avoid that situation at all costs). I have a justified reason and it's totally ok for me to not be well enough to work right now... but there is that worry that people won't understand, that they will conclude you're lazy or not good as good as them. I would go as far as to avoid seeing people so I didn't have to talk about "what I'm up to". It was my nan's funeral last week, I never saw her really or that side of the family, and my anxiety is so high that I couldn't face the amount of people who were there --- and a big part of that was because I couldn't handle the small talk, the "how are you"s and the "what are you doing these days". I feel inferior to EVERYONE just because I'm not working...
I can't offer any advice because I'm struggling with that too, but I do know that we ARE good enough and that there's no shame in being ill. If you had a phsyical illness you wouldn't feel bad for not working, and it's just the same really.

Anyway, congrats on getting through the vacation I hope you were able to sort of see that, yes your sis in law is "sucessful", as are other people, in some ways but is she really happy? Is she fun to be with? Nobody is perfect, it's absolutely true.