Would God be upset if I had a 3some?

I grew up Catholic and God has always been first and foremost. I have tried to follow His way and do right. But I know God is also forgiving. I am extremely close with my sister in my house. We do almost everything together and I've known her forever. She has a very hot BF who I know for a fact also likes me. I been toying with the idea for months to have a 3some with him and my sister so I don't betray her trust. The idea has been driving me crazy. I know it's soo wrong. BUT I know God can forgive me. How wrong is this and what advice can you give me?