Where to take my cousins (they don't talk much)

I would have suggested any shopping areas, but since you mentioned that they don't like malls, perhaps any sporting events might work. It may encourage some conversation as you watch a game together in a loud environment.

Otherwise, I think you should just try to start conversations with them whenever you have the opportunity. I've met many quiet cousins, and by the end of one week, I was able to open them up a bit more.

Jam it back in, in the dark.