My nephew, a chinese-mix, starts to be able to walk properly and support himself ard 18-19 mo, I think your niece just a little slow on that.
As many has suggested, she might be a bit lazy or lack of confidence. I think the parents should "encourage" them and try to walk with them more often to kick-start the child's motivation to walk by himself/herself.
Another thing, does her parents always carry her everywhere she goes? Maybe that's the reason your niece still can't walk on her own? Just a though.