I agree with Bath and Pixie. I think in that situation, while her reactions are hard to fathom, you just have to feel sorry for her that she is so wrapped up in the past and so lacking in compassion for her sister that her life would surely have been the sourer for it.

DP's family has lots of stuff going on at the moment and TBH I just end up feeling sorry for the lot of them. Sorry for the BIL and the SIL who can't stop *****ing about the other brother because it really is affecting their lives because they can't let it go - and sorry for the other brother that he can't swallow his pride and apologise for his bad behaviour.

In some ways, feeling sorry for people is kind of a superior attitude - "oh you poor things, if only you could handle things as well as I am because I have more going on in my life than worrying about who did/said what" - but on the other it WORKS.

I think that perhaps you could delve into her emotions a bit more but in a very, very calm and detached way. That could lead to a better understanding of her. On the other hand, you could just find an internal way of not being bothered. There's no right or wrong answer.