Hi M.,
I've read it can be absolutely normal for some boys to bed-wet until later in their life, even 10 year olds. That's why they made "cool" boxer-like sleeping underwear for them to wear at night.
They say that boys bladders sometimes don't grow until like 12-13, and the result is bedwetting. I remember when my 2nd son was 3-4, and we were on roadtrips, he couldn't hold his bladder AT ALL!! We had to stop all the time for his little bladder :o) My older kids were not like that at all.
I don't think your nephew should be made aware of your concerns. He's at an age where he could be mortified.
I think his mom should talk with a doctor/urologist, to ask the simple questions first to decide if your nephew should even be seen.
In the meantime, you are a very caring auntie who should probably be careful with the way you approach your sister about her son. You don't want to stir any "waves". You might just have to leave it all alone and let the problem stay within their home :o)
Good Luck!
~N. :o)