My dearest Brey,
Always remember that family is forever. Yes your cousin should not be engaging with someone that you are interested in, but keep in mind neither of you are married to this young man or even in a committed relationship with him. So truthfully he is free to 'talk' to whomever he pleases.
Now think about this - would you want to be with a guy that in only 2 months is playing Jedi mind tricks??? I would venture to say that you DO NOT. Here is how you solve the problem. Talk you your cousin and let her know you are fully aware of what is going on. Tell her that you care about her and that you don't want someone outside the family to ruin your relationship. Also tell this guy that you're not interested in someone who will blatantly disrespect you and your cousin.
Sweetheart you need to realize that you are worth way more than that. No one in this world deserves to be made a fool. The only person that can put a stop to this is YOU. You need to make a decision and live with it. There are over 7 BILLION people in this world don't allow one person to cause to you 7 nanoseconds of stress.
Good luck lil lady and have a great weekend.
Erika