Hey L.,
I know this isn't your child, and I hope this isn't too presumptuous, but I feel compelled to write as I had the same problem when I was young. As your niece is well past the age when one would expect for her to be experiencing this kind of problem, I would ask if anyone has taken that little girl to a really good urologist. She may have a physical condition that causes these night time accidents. I did when I was young, and if my parents hadn't had it checked, I might be on a dialysis machine today. My bladder had grown faster than the rest of my body (not uncommon for some body parts to get a little ahead of the others,) and it was affecting my kidneys to the point where I was required to take antibiotics daily for 8 years. Perhaps your sister or brother has already talked to a doctor, but if they haven't received a referral to an urologist, I strongly suggest they do. In any event, she should be checked for the rash, definitely. You have no way of knowing what you are dealing with, and it would be a shame to give her the wrong thing based on guesswork. If her problem isn't physical, then maybe there's a possibility that something is going on psychologically. This child's parents may be oblivious to these possibilities because it is there daughter and they have accepted this as the norm. But it is BEYOND NOT NORMAL, and if you feel close enough to them you might consider having a discussion about it with them. You'd be doing your niece a huge favor.
Good Luck,
A.