I've masterbated with my best friend's sister.

I moved in with my friend and his mother and sister b/c I needed a place to stay and his mother needed some help with the bills.
I've masturbated in front of my best friend's sister before. I usually pretend that I don't see her. I am 20 and she is 16 going on 17. We dont touch one another!!!! Since I started letting her walk in on me changing a few times. Or let her think she is spying on me by leaving the blinds up or leaving my door cracked just a little bit. Some times I wear very loss and revealing boxers so I can catch her looking. I have accidentally walked in on her changing or masturbating since all of this started. I feel bad b/c this stuff only happens when her brother and mother are not at home, sometimes late at night too. Sometimes we start to wrestle and I get a boner and have to stop and try to hide it. She very blatantly makes her lower region rub on my groeing.Is this normal. I get such a huge thrill out of all of this. I really dont know what to do about this???????