Basically my parents and my younger sister have fallen out. The history is that my parents are pretty devout catholics. We were obviously raised Catholic but have all pretty much ditched religion entirely. One brother and his wife are half-hearted catholics - their kids go to Catholic school, they go to church at easter and Christmas etc, but they're more modern in their beliefs around divorce, abortion, homosexuality etc than my parents, who are very traditionally Catholic in their views on these things. The rest of us are atheists.
I obviously dont agree with their views and have said as much many times, but mostly I just roll my eyes when they say stuff. They're old and set in their ways, they are wrong to judge of course, but they don't actively harm anyone with these beliefs.
I'm reasonably close to my youngest sister, we meet infrequently but message a lot. Out of the blue (to me, anyway) she's announced she wants no further contact from my parents and she is cutting them out of her life due to not agreeing with their beliefs. My parents are very sad. I feel upset about the situation and think it will drive a wedge in the whole family and make family events like Christmas and weddings really awkward or force people to choose between who they spend time with. So I feel sad about this but can't really apportion blame to either party as I can understand why my sister doesn't tolerate their views.
So that is the back story. The aibu is: my mum asks after my sister. I've said I will tell her a general "she's well/she's happy/she's fine" but I will not betray my sister by giving my parents personal details about what my sister is up to. My mum accepts this but of course is sad. However, my sister has always slagged my parents off to me. As in, we would both moan about the things they say and roll our eyes etc. I met up with my sister for the first time since the rift today and to my surprise, she immediately said "so what crazy shit have mum and dad been saying lately?" I was taken aback that she thought the old dynamic would continue, so i just changed the subject. She later asked again and started ranting about something they said years ago that we have discussed before, as if she wanted to drag me in to moaning about them. I just said, "Why do you need to know if you don't want them in your life anymore?" She was very angry and ended up leaving. I've messaged her to say I'm sorry she's upset but that I've told my parents I wont discuss her with them, so it's only fair the courtesy should go both ways. I honestly don't understand why she wants to spend the afternoon moaning about them when she says she's cut them out of her life. She replied saying I've made it clear who's side I'm on (ie, my parents) and that I'm a hypocrite for changing my opinion on them. I tried to reply that my opinion hasnt changed, but She's now blocked me 😔
This isn't about the difference of beliefs or whether you think my sister is right to cut them off. My aibu is, am I unreasonable to say I will be neutral between both sides and not pass on information about either?