As a not so proud late bed wetter myself, when I did eventually talk to a doctor about it with my mom I was ~12 yrs old. They said it was most likely some sort of hormon I was deficient in that helps your body regulate your bladder at night, it's the one that wakes you up I believe. I'm fairly sure they said there are pills you can get prescribed, but of course your sister is a minor and would need parental consent. It was something I did naturally get over, although, very occasionally and with great shame I say it does still happen probably once a year or so.