My daughter is 10 and still wets the bed. She was fully potty trained both day and night and in reg underwear by 20 mths old. At 2 1/2 she started having daytime accidents when her father left. It lasted for about a year. Then she was fine. Then she started again when she was 5 when her father re-entered her life and mine. It lasted for a few months then stopped. Then when she was 7 1/2 she started wetting the bed at night. Something that never happened before. Her father left again when she was 8 and the amount of bedwetting increased to the point that some nights she wets 3-4 times. She never wakes up during the night. I've stopped drinks, food, woken her up at night(which she never remembers), but it still happens. Nothing has worked. She'll be 11 in a few months and the bedwetting has continued. Every few months she has a checkup with the doctor over it. We've tried every medicine there is for it and none of them worked for her. The only thing I haven't tried is the bedwetting alarm cause the insurance company won't cover it and they can get kinda pricy. I've been told that's the last resort. She's been using Goodnights for years now. Although some night she leaks really bad cause they just can't hold all of it. So besides having a fitted plastic sheet on her bed under her reg sheet, she also uses washable bed pads like they have in the hospital. The doctor has done every test possible but nothing medical is wrong. Her bladder is fine, it's not underdeveloped or too small. He says she will grow out of it. Within the past few months she has had occasional nights or even as much as a week without wetting, but it returns full force. For her it's been mostly emotional based, but she is also a deep sleeper. Sometime these kids just can't help it. Some get very embarrassed about it. In my daugthers case she doesn't really care and isn't very embarrassed that her friends know or that she wears her Goodnights at occassional sleepovers. It does worry me that she doesn't really care, especially with her going through puberty now.
Your sister-in-law should at least take him to reg doctors appts just so the doctor can have a history on it. And also to make sure there are no underlying problems that need to be addressed.
D.