Is it normal that I had sex with my ex boyfriend's sister's ex?

Well im not a lesbian but I ended up having sex with an ex boyfriends sister once.
It was an emotional time and I was close to his family and always hung out with his sister. When he broke up with me she stopped talking to him and was there for me as a friend. We got terribly drunk one night and ended up getting it on together.
We woke up the next day naked in bed together and had another go at it now that we were sober.
She was a better kisser and lover than her brother.
We dated for 6 weeks but as we are not lesbians it started to get a little weird. We got together a few times after that over the next couple of years but she has moved away. She comes and stays every now and then and we cant keep our hands off each other. She is coming next month and we have already started sexting each other so I guess we will jump back into again when she comes to stay with me. Im not attracted to any other girls, its only her and I love her so much.
OMG I never gave my own situation so much thought before. When I think about it, it sounds screwed up but I actually cant wait to see her again.