Would you believe that last weekend I went to a concert and sucker punched some random guy in the crowd for no reason?
Are you joking? I hope you don't mind me asking if it was really random or did you have a motive behind it.
Honestly I did have a motive. That guy raped my sister.
It's very understandable that you want to physically attack the person who raped your sister but using physical violence against him is still nonetheless wrong. Instead of taking justice into your own hands you should contact the authorities so that the person can be tried and punished by the legal system for the sexual assault he committed on your sister.
Screw contacting the law. I want to make him pay.
Wanting to make him pay for what he did is fine but you should do it legally. Attacking him may just end up with you being the one getting punished by the legal system and him getting off with just a punch or two from you. I'm really sorry to hear about what happened but you still shouldn't be trying to physically harm the guy.