Would it be too horrible if I tell my stepdad to not interrupt me when I'm talking?
Does he interrupt you a lot?
All the time. He doesn't know how to talk nicely to me.
I am in agreement with you here. Interrupting someone while they are talking is rude and disrespectful to the speaker.
I'll break his leg if he interrupts me one more time.
Well physically hurting him isn't the right thing to do in this situation. Just asking him politely to stop interrupting you should get the job done.