Do you ever see this niece in person? When I made a quilt for my boss and coworker as a surprise I paid attention to what colors they wore or if they showed me pictures of their home for the colors. I also tried to pay attention to what they talked about. For example, my coworker is a crazy cat lady just like me. We both talk about our pets a lot. And when she was buying a new couch she talked about it. The couch was leather and she liked cats. So I made her this quilt:
Buckeye Beauty in cat prints She loved it.
My boss is the only woman in a house full of guys. Even the dog is a boy. So I wanted to have something that would appeal to just her. I notices she wore a lot of turquoise and coral colors and I wanted something kind of flowery and feminine. So I made her this:
Triangle quilt
Or do as aashley333 suggested and ask them for a single color they like or maybe ask what the wedding colors are and take it from there.