I have a dilema people please answer!!!!?

well my cousin was talking to this gurl he said he really liked bt in actuallity he didnt wanna b with her because he wanted to tlk to and have with other girls while still having her on the side. he lied and lied to her about so many things and one day she asked me for advice me being the totally opposite from my cousin i gave her advice that didnt make her walk away from him but at the same make sure she kept her piece of mind. after a while i noticed that she talked to me more and more and my cousin even though he was messin around with other girls started to get mad at her for talkin to me. he would constantly asked her did she have feelings for me and other stuff. well skip to two months later i am now dating her and he is so mad bt he doesnt confront me about it he does things like read my text messages when im nt around my phone and try to get into my myspace to see what im saying to her or people. i really like her bt shuld i stop dating her because of him.