My underage sister with a man over 20...

My sister is 15 and this person is over twenty years old. I myself am 20 and I was highly disturbed. Especially when I found out he had sex with her when she was 14.

This is just hard to deal with. My sister has bad depression problems and my mom is trying to deal with the situation because we think it's wrong.

I've been in her place before and I know how it feels, but we think it's wrong and most of all it's illegal.

My mother plans on going to a child advocacy center to see what else we should do, but we are all in a way confused. My sister felt sge loved him, but my mom and I think it's gross. Especially since he was clearly manipulating her feelings by the way she described it to me. What do you think?