My parents were foster parents and while is has nothing to do with abuse, neglect or income, having a lot of children go through their house meant bed wetters were part of life. My adopted sister was really bad, and like your Sister in Law they almost went with "an expert", at the going rate of $1500 dollar then...such was their frustration with my sister and bed wetting.
Here was the thing though...
The bed wetting expert brochure had this beautiful, fancy, multi-storied building on the cover...like you would see at a Doctor's Complex, YET there was no sign out front stating what the place had anything to do with the company. They concluded it was a scam because it would be like me going to a sheep farm in New mexico and taking a photo of thousands and thousands of sheep, and making it a brochure for my sheep farm here in Maine...misleading to say the least.
I am glad you SIL did not spend all that money for the expert, and I am super glad the child grew out of it.