My boyfriend's cousin is also my ex and so we've have a past with each other. And he's also cheated on me before so I'm really confussed at what I should do. And as months passed we've grown to be friends again and make a friendship. So what should I do. I'm 13, my boyfriend is 15 and his cousin is 13

What do you mean by cheated?

What you should be doing is going to school, hanging out with friends and not be worrying about a "relationship"

Learn more about life and how people act and how you should be interacting with the boys and the girls.

Be friends with them and a lot of others too. Don't try to grow up too fast. Take this time in your life to have fun and learn about life.

You will be grown up before you know it and you will regret not taking the time to be a teenager.

At your age, I don't think you need a boyfriend to begin with. Since you are confused about what to do, just be friends with both of them then you don't have to worry about one getting upset or your being confused.
For future reference, it's not a good idea to date within the family. It could get sticky

I'm sorry but at thirteen you should be thinking about school and talking with your friends.

You be careful or you'll have to grow up too fast.