J.,
It is not completely uncommon for your niece not to be walking, it is within the range of normal. How is her other development? Is she pretty much doing everything but crawling? Is she cruising? Is she stacking blocks etc? If everything else seems to be on track she might just be afraid to take the plunge so to speak.
Did she do other things later such as rolling over, sitting, crawling? If so she may just need more work to build her core muscles. Especially, if she didn't have a lot of tummy time when she was first starting out.
I would also suggest a visit to a chiropractor and/or a cranial sacral therapist. I can give you referrals if you are in the Jacksonville area. Sometimes children are injured at birth and a slight adjustment does wonders or if she has chronic ear infections this could possibly cause her to feel unbalanced and thus delay walking...chiropractic adjustments have saved many from tubes and life long vertigo problems. I have known at least two or three people that have seen dramatic results from late walkers through craniosacral therapy. I know these are considered alternative treatments..but much less invasive or risky than a battery of tests on a baby.
What type of birth did your SIL have? Was it vaginal, or c-sections? Were forceps/vaccum used? Was she induced? If so then your niece could have been born prematurely and that might explain some of it.
Has your niece been vaccinated? If so she could be suffering from vaccine damage. So children react to the preservatives in vaccines and sometimes the vaccines themselves. Have they noticed changes in her behavior after she has had vaccines?
What sort of diet does your niece have? Is she still nursing? You may have them really examine her diet and make sure she is getting plenty of protein and essential fatty acids. Remember milk is okay but too much could cause problems. Foods rich in fatty acids (EFA DHAs) are fish and eggs. And of course fresh fruit and veggies. Staying away from processed foods is not always possible but the better choice.
It is probably likely that your niece is completely normal and there is nothing to worry about...I am just offering these things as suggestions if you find there are things to examine. I would be glad to speak with you in more detail if you have questions, just email me. HTH, S.