I'm going to force my cousin to stop vaping
Why do you want to make your cousin stop vaping so much?
I don't care about his opinion.
Vaping is just as bad for your health as smoking and you do care about him. He is old enough to make his own decisions so let him. If you try to make him do things, he will rebel and do it even more.
Nah, I don't really care. I'm going to make him stop.
It's good that you care about your cousin so much, but do you really think that you will be able to force him to stop? You may end up ruining your relationship.