My deepest condolences on the loss of your mum, mosaicwarts, even though it was a long time ago. I'm deeply sorry, too, that ESIL (estranged sister-in-law) disposed of and destroyed so many of your mums things w/o giving you a chance to go through them, read the diaries, etc. Having a deceased loved one's diary, especially, is like having a piece of them, IMO, and I'm very sorry you didn't get the chance to decide if you wanted any of your mums.
Since ESIL and EB owned the house along w/ mum, however, they may have had the legal right to do w/ the contents of the house as they chose. IMO, they could/should have been more sensitive to your feelings, but legally, they may have had the authority, after all. I don't really know.
I'm saying "they" even though you seem to be putting all the blame on ESIL. I can't help but think EB must have had some part in all this, too, especially since you're CO (cut off) from him, as well. But I know I could be wrong.
I admit, I don't get burning the diaries. Is it possible mum asked them to do that b/c there were some very private or potentially embarrassing comments in them? Or perhaps there were some complaints about ESIL in them that she (ESIL) didn't want anyone to see (though, in that case, IMO, she should have sucked it up and let you read/have the diaries if you wanted them).
IF they were legally allowed to dispose of your mum's possessions, then I don't really see where they mistreated HER. I DO think they mistreated YOU, in a way, as I said, by not being more sensitive to your feelings. And again, I may be wrong, but I suspect it's your own hurt that caused you to cut them off (not sure why you CO your dad).
Regardless, I understand that it's uncomfortable for you to maintain a relationship w/ your aunt since she also has a relationship w/ EB and ESIL. As others have said, though, perhaps she values maintaining all her family relationships despite disagreeing w/ some behaviors. I'm sure you realize that's her right. Hopefully, she doesn't discuss her relationship/visits w/ them, etc. w/ you or vice versa. If she does and you would rather she didn't, please let her know.
Also, I suggest you make sure she has all the addresses, etc. she needs in an accessible place, so that her POA friend won't have to ask you for them again in the future. The point, IMO, isn't to change the fact that aunt has kept up w/ both you and your EB, etc., but to make sure that this situation isn't awkward for you. Hoping it all works out for the best.