Does she "pull-up" yet? Does she "stand"? Can she "stand" assisted or not? Can she sit-up unassisted?
If she is 1 yr. old and has never crawled...well some babies don't crawl. But, in this case, since she is already 1 year old... it should be discussed with their Pediatrician... to rule out anything.
Is her overall development "normal?" Does she talk yet or do other things that ARE age appropriate? Had she met her other "milestones" at the appropriate age? All of these things needs to be considered, in light of her seemingly inability to stand/walk on her legs. You mentioned that her legs do NOT seem to be able to bear any weight on them, and she does not use her legs much... so I would really get this looked at. It could be nothing, but it could signal something else.
Since you mentioned also that her legs seems to "buckle" under her and then she just sits down... I would bring this up to the Pediatrician. It seems puzzling... I think you are not over-reacting... but it needs to be brought up at least to a Doctor. At this age, a child SHOULD be able to "stand" up... and bear weight on their legs, and at least use furniture to "walk" along it.
You might want to look up developmental milestones/skills that a 1 year old should be hitting... a good website for this is:
www.babycenter.com Look at it and see if your niece is on par, in light of the inability with her legs.
All the best,
Susan