My BIGGEST regret

My boyfriend, Luke, and I have been dating for around a year now. He has a twin. We visited his family for the summer and his twin brother was there, everything was fine until the day where my boyfriend left the house to go shopping without telling me. I thought he was still in the house with me, and since no one else was around I decided to put on my sexy lingerie that i had packed for this type of occasion to surprise him. So I found him in the kitchen and started getting close to him and all, and we were in the middle of getting it on when he says "God, I'm glad Luke is out of the house for a while." and then I realized that I am having sex with his brother! but the worst thing is that we didn't stop after that... I feel terrible, I shouldn't have done this especially after I found out it was his brother. And now I am 3 weeks pregnant and I don't even know if its my boyfriends child or his brothers! At least they won't be able to tell easily... But i still feel guilty, what should i do? Please I need good advice!