hello, welcome to the forum
first of all i think you should have talked to your nieces before ordering the belts. how did you take the measurements? what did you tell them
well, now the belts are already there, but forcing or pressuring is always the worst option. first i would talk to both of them, maybe you can convince them with your arguments, if not you can of course try to talk them into a deal, so that you give them something in return if they wear the belts.
and no, legally you are not right, you must not legally force them
Jenna I did not realize the legalities part, and this is not something I can openly discuss with the authorities. I was under the impression that they are under my control until they were 18?
in certain areas you have control, but not when it comes to your niece's body. You can't hurt her on purpose or anything. Just because they are minors doesn't mean they don't have rights
Jenna I am hoping for an outcome where all sides agree.
That's OK in my opinion, but you should accept it even if you can't find a way to get their in the belt