I want to take my niece to Disneyland and I'm mad at my brother and his fiance for saying no. This is something I think should be up to her soon, right?
Why do you want to take your niece to Disneyland? How old is she? Do you really think it's okay now?
Because of her being so cute, I've waited many years to take her to Disneyland when she turns 18.
It is nice to want to do things for family members, but you need to respect their wishes. Your last sentence also makes me wonder your motive for taking her and I think that your need to evaluate that before you get yourself into major trouble.
I'm going to take her to Disneyland anyway. I am pissed off at my brother for his trying to wrong me for wanting to do something nice for his child.
You should respect your nieces' parents wishes. However when she is 18 she will be old enough to decide for herself, just don't get weird with the cute stuff. Until then try and respect the wishes of her mother and father.