I dont know what to do i feel horrible!?

This is not your problem. You can't be responsible for your mother in law's behavior. She is to blame, not you. You have a sick child and I"m sure they appreciate you not coming and spreading the germs. lol Get her a gift and call her on the phone and tell her how much you love her and you will miss her, etc...and let it go. This is not your issue. Your mother in law is responsible for getting out of denial about her son and finding a way to try to help him.