Should I tell my dad to stop talking to my mother in law?
That's a strange thing to ask. How would your dad feel about you asking him that?
I know my dad is going to be mad at me for asking this, I just want to have him on my side.
You shouldn't tell someone who they can talk to. Your father is his own person and can talk to whoever he wants.
He's the one abusing me by talking with my mother in law.
It does sound like you are having trouble with them talking but you cannot and should not stop them from talking to each other. They might get upset and stop talking to you instead.