I am sorry you are allowing your cousin's behavior to upset you. Remember we cannot control the behavior of another and if you feel strongly about his behavior you have a couple of choices. First of all you can just cut ties with him and not hang out together as he is not behaving in a manner you can tolerate so move on and let him be to live his own life.
Or you can be assertive and talk to him about how you feel. I am not sure at his age and with his behavior that he is going to be open to discussion with you but it is your way of letting him know how you feel. Just don't expect that he is going to change.
Now for girlfriends, it feels to me like you are over thiniking it. Just relax and be yourself. You do not have to be the sounding board for your cousin's girlfriends or feel it is your duty to save them. Concentrate on living your own life and maintaining your self esteem. You are a good person so please believe in who you are. Whenever you start to feel that self esteem slip, remind yourself..............."I am a good and kind person"
I hope you find some help in my words and that you will move on with your own life and not feel responsibile for how others live their lifes.
"You are not the momentary whim of a careless creator experimenting in the laboratory of life. . . You were made with a purpose." ~ Og Mandino ~