Cousins often look to girls they know well to learn more about the female body.
The answer depends entirely on you and your cousin. Some cousins have absolutely no interest in experimentation. If you try to experiment and they say no, it means no.
Other cousins may be perfectly fine with playing around a bit as you learn about each others bodies. Ask them what they are comfortable with. When you are too shy to ask, go slow and see what happens. Try touching a bit through their clothes. If they are upset, stop. If they spread their legs and let you touch, keep going.
Some cousins will want to experiment to the point of seeing each other naked while others will want to stay in their clothes. Occasionally, cousins even end up having sex with each other. If you do, make sure you use protection so no one ends up pregnant.
No matter how far you go with a cousin, make sure you are both ok with whats happening. It won’t last forever – its just experimentation – so enjoy it while it does last and be prepared to move on to a real relationship as you get older.