Please help - family problems?

What your parents do is really none of your business and you shouldn't be involved. In this situation I think it would be wrong to tell your dad about your mom's boyfriend. He is very sick so what purpose would it serve to upset him? Your parents relationship is between the two of them so you really should step back and stay out of it. I think it is a good idea for you to move as soon as you are able to because if your mom actually would say she would kill you shows she is not a great parent and you would be much better off out of there and away from her. Bide your time, finish high school and then leave. In the meantime just try your best to ignore what's going on between your parents and let them do what they choose to do.