Marriage is not a game. It’s a lifetime commitment. You can’t marry her just because you were in a relationship with her. You need to be 100% sure about this. Many people come and go in our lives. We might have romantic relationships with some too. That doesn’t mean we will marry everyone.
Have a good conversation with your friend. Try to know why he wants you to marry his sister. He can’t just threaten you to marry anyone. Try to talk to his sister too. Ask her does she also wants to marry you. If yes, then why? You had a past together but currently, you are not. Then why does he want you to marry her? Make him understand that marriage can’t be done forcefully. Don’t be confused about it. Ask yourself do you still love her? Do you really want to marry her? When you get your answers, take action accordingly.