I've never understood why people become so callous after the deaths of loved ones. It's like the worst comes out in people, mostly because of money.
As for the sister-in-law, I suggest you do stay away - far away. Why would she say you hit her? I just don't get it.
I would try to get out, and stay away from such toxic family as all they seem to do is make you feel bad. I've done this myself, as my mother's side just totally depresses me. I've made friends through finding work to occupy myself and get me out of the house.
I hope you find friends and caring people you deserve. It sounds like you need it. I hope there is someone you can reach out to, as it does help during times like this.