I'm worried I think I dislocated my niece's wrist
That's scary. Why do you think you dislocated her wrist?
I grabbed her hand and felt a pop.
It's irresponsible to play with kids in an aggressive way because they're vulnerable to injury. You have to be careful when you're playing with your niece, so that she doesn't get hurt again.
It's just her hand. How bad can it be?
Just because it's a smaller body part, doesn't mean it isn't important. Children are very fragile and the hands are extremely important. How would she feel if she found out you were more rough, and her hand actually broke?