I think my niece needs to shave her arm pits.
Why does she need to shave her armpits?
I think she is too old for her to not shave her armpits, and it is unattractive not to.
There's nothing wrong for her to not shave her armpits. As long as she is not smelly or there are no underlying health issues with her having armpit hair, you should not have problems with her having armpit hair nor you should tell her to shave it.
Yeah, I think that all girls should shave their armpits. Besides she is a girl, what other grooming do can expect from a girl?
While you may dislike the smell, there's not a whole lot that can be done about this without being considered rude on your part. Other people are expected by society to be allowed to do what they want with their bodies, and we are expected to respect that.