wow that is some tricky situation!
i would say you can speak to your friend and tell her that this is a complicated situation that she is getting herself into. however, i don't think this is something that you should be bothered about. i get it that you are feeling guilty towards your SIL, but at the end of the day, it is your dewar and your friend's decision, and even though you are somehow connected to everyone here, it is better you stay out of it.