Is he lying to me ?

Ok help me out here, my livin boyfriends, sisters ex calls me and informs me my boyfriend is cheating on me.. So i confront my boyfriend on the phone ( i am in another state at the moment) and he says it is a lie, that his sister broke up with this man and now he is trying to start trouble between me and my boyfriend. I want to believe my boyfriend for the simple fact that he calls me almost every ight and tells me he loves me. He also asked me to pick out colors of paint for our home. So tell me what you think please. I have major depression and am out of my meds. This is really getting to me. The informant is a drunk and he keeps calling me and then today he tells my man i am starting all this mess!! No i didn't..