Please help - girl advise?

Okay so during winter my brother (15) has a group of littler boys my age (all one grade younger than me) called his 'minis' and they snowboard with him. One of the minis is a boy named Cole. We started dating two weeks ago and we broke up because someone else liked him, more than me!?! and we got back last week! But people are trying to make me break up with him because he's cheating with 2 other girls. He was dating them (Zoey and Grayce) and then he stars dating me! Is it wrong that I told the people who tried to make me break up with him that it's their problem, not mine? I think it's because he's cheating on them with me, not me with them.Is that wrong? I don't know. We are still dating but I don't hardly talk to him unless he's on facebook. Well....help would be nice....