I am not sure what the diagnosis is but I would recommend an evaluation by a team of professionals at a local university or hospital to identify or rule out developmental issues. The advantage is that children with defiticts profit most from early diagnosis and intervention. And if there is nothing wrong, then it is great to rule out problems. the diagnosticians should include a developmental pediatrician or a pediatric neurologist as well as PT/OTs and language specialists.
I have a 4 year old autism spectrum girl. She did not walk until 16 mo...and is behind in other ways. It really sounds like your niece does have some issues that she should be seen for. It could be mild autism spectrum, or a learning disorder, or ?? Sometimes parents are so traumatized by thinking that their kid could have autism or something like it, they are in denial....we were like that and very angry when it was mentioned to us a long time ago. Now, we talk about it openly. My advice would be to mention a book or tv show that you have seen about autism spectrum and see if they want to talk about it. Otherwise, she should be seeing a developmental pediatrician for help. They say that age 0-5 are the best years for treatments. Hope you can talk openly to your sibling.
-Angi