As a mom to a TEENAGE bedwetter (my daughter is 17) my suggestions are as follows:
Yes, some do grow out of this stage (though not in my case) - and other do not - Your sister/sister-in-law should take her to see her pediatrician to discuss the matter - there may be a medical problem lurking.
She can use pullups - or goodnites in the meantime.
If there isn't a medical problem - it could very well be stress related.
IF the pediatrician doesn't seem to know what to do or if your niece's mom is not satisfied with the results - she could seek out a pediatric urologist to see if they have any suggestions.
I don't believe in the nitetime alarms if your niece is such a heavy sleeper - in my experience, the alarms only woke everyone ELSE up in the house and not the bedwetter, and they corroded after a month or so.(even after thorough cleanings)
I also don't believe in waking the child after they've been sleeping to go to the bathroom - if they are in such a deep sleep and you are 'dragging' them to the bathroom and making them go, chances are they won't remember going the next day. (in my experience this has happened)
I've used the following 'techniques'/'rules' :
no food/drink after dinner - eating promotes drinking - drinking promotes bedwetting.
I've had my child sit on the toilet for 3-5 minutes every 2 hours - even if she said she didn't have to go. I would give her a puzzle or her gameboy to occupy the time - the time spent on the toilet allows for her 'system' relax enough to go. I've also bought her a watch with an alarm and taught her how to set it to go off every 2 hours to remind her to go to the bathroom.
The last resort is the one I had no choice in making and that was to put my daughter on medication. DDAVP comes in a nasal spray and also a pill form. Ditropan is another medication in helping with bladder problems. She is on both and I'm happy to say hasn't had an accident in a long time! We've tried to take her off the meds only for her to have an accident and then go back on them. The doctor's say that her bladder is just immature and there's no telling when she may stop.
I hope this helps....good luck to your niece!