Why does my 7year old sis still wee the bed:S !?

my little sister is 7 years old, and she still pee's the bed. ive noticed quite alot that she only pee's the bed when she goes to bed early this would be at 9 on weekends, and when she stays up late watching tele with me and we both go to sleep at the ame time which is around 2 in the morning, (bad yes i know) lol, but thats when she never pee's the bed. she doesnt have a peeing problem atal, but this has become and issue because i have to share bedrooom with her ! and i need to know whats causing this and how it can stop because the smell just eww! its rank and it brings flies lmao, hope u cn help. (OH AND BTW SHE GOES TO THE TOILET 2 EVERY TIME BEFORE SHE GOES TO BED IF THIS HELPS)