Boyfriend making me choose

My boyfriends 22 I’m 17. We’ve been talking for a long time and recently got together. When he was having a hard time ( which seemed to be a lot of times) I was there for him. I opened up to him when I wouldn’t even open up to my family. Anyway he got in a fight with my sister now he wants me to choose him or her. Both have hurt me a lot but she’s my sister I’m lost and I don’t want to lose him but I know what he’s doing is wrong I just don’t know how to handle it