MIL has been horrid to me for years. I have taken it personally, although have come round to realise that it is not my fault, I have tried my best to be a good DIL, and it is her problem. I have good relationships with others, don't tend to fall out with people and she does.
Anyway, DH has been telling me that she really isn't of 'right mind' for ages. I think I finally believe him. He told me the other day that he had a conversation with her where she asked him to promise that, if she died, he would cuddle her dog and get a vet to put her dog down while he did this, as "noone could possibly love and look after the dog as well as she does." 
This is not how a sane person thinks, is it? DH really is right, isn't he?
BTW, of course, between BIL and us, we would make sure the dog was well looked after if something happened to MIL.