Please consider this as an opinion and advice. Think over it.
Right now you are 13, and I understand that you might be confused in recognising your cousin's feelings as well as his intensions. He is 15 and knows better than you about how he feels and what he is doing. I don't want to scare you but this might be a case of premature affection and might result in molestation. These feelings might just be your harmones (those chemicals in brain). So don't go ahead yet. And don't let him touch you the way you just explained here. It's not good for both health and your mind at this age. Do not get in physical or emotional relation just yet, it messes up with your overall growth at this age. You are 13 and I think it is asking a lot for a girl your age, to think rationally and decide for yourself but, considering my advice I hope you choose well.