I like him, he likes me, but he dated my bestfriend?

I dont control who i like and i never even ever though of liking my bestfrineds ex- but i do. 4 months ago, my bestfriend was dating the guy who loves me and who i love aswell. Hes asked me out, but i had to say no because hes her ex-. He told me that their done but she said she'd be mad if we did date. Its not like i can control who i like? But my sister talked to her and asked her if she'd be mad if we dated, and she said she would be. I know its wrong and that it'd be backstabbing. But she got over him, and started dating this guy who she's been going out with for at least 4 weeks. In your HONEST opinion, would you be mad? What would you do? What should i do!?