I don't want to play computer games with my cousin
Do you not like playing with your cousin?
I don't want to play with anyone besides my friends
Choosing who to play computer games with is your decision in the long run. But if your cousins want to play then they should be allowed to, they are still your people. They might feel hurt and cry themself to sleep at night if you don't let them play.
I'm only doing this because it's not worth the headache from them being upset.
It good to be nice to your family. One day you might look back and be glad of the time you spent with them.