I'm going to warn my cousin about buddhism
why do you want to warn your cousin about Buddhism?
Because I know he is not open-minded and wouldn't understand
It's not ok to talk bad amy religion. If he wants to follow buddhism, he has all the rights to do so. You shouldn't tell what he should do or not.
I'm his cousin and i have to look after him. He doesn't know what is best for him.
There is nothing wrong in buddhism is that you need to warn your cousin about. Also, let your cousin study about it and decide if it's good for him or not.