Help!! Idk what to do

Ok. There's a guy. I've liked him for the longest time. We've started hanging out just the 2 of us. We're constantly talking. He's only been with one girl but he still kind of a player. We were talking yesterday. I wanted to know if he was only talking to me bc he wanted to sleep with me. He said no (thank god). But there is a problem. First my sister is dating his brother. And second the girl he was dating is part of our group of friends. After he broke up with her they were friends with benefits. I asked him the last time they had sex and he said it had been months. His ex is still in love with him. She told my sister that she'd wants be friends with benefits for the rest of their lives rather than find someone else. Which is crazy!! His ex told my sister that they were still having sex and I don't know what to believe. She really hates me bc I like him but I liked him long before they got together. He's told me he likes me too. It's just really complicated. Idk what to dooooo. Help me please!!