I physically attacked my father cause I thought he was kidnapping my sister
Why do you think he was kidnapping her?
I thought he was a stranger at the time.
It sounds like you had a big misunderstanding and you are wrong to physically attack your father. You should give your father the benefit of the doubt.
My sister has the mind of a 12 year old. She would do anything he says.
Well that's not an excuse. You need to make sure someone is actually a stranger trying to take your sister before you hurt them.