Dr. Gould is definitely right. Just like binge eating, binge drinking (water) is a way to clear your mind of what the true problem is. When I would go study for an exam in college (I recently graduated), I would take at least 5 water bottles with me. The good news is, rarely does anyone die from drinking too much. The truth, though, is that your cousin could have something medically wrong or/and psychologically bothering her. Just comfort her and give her an example, like mine, so she knows she's not alone. When she's drinking encourage her to stop and take a walk...divert her attention from the binging so she can understand what she is blocking out (mom, school, happiness). She'll have to make the decision to admit reality. And maybe there is something medically charging her to drink as well. Good luck; be strong.
she needs to see a medical doctor asap. Drinking that much water may be a sign of diabetes or adrenal problems
take care
I do know what diagnosis fits her, and it doesn't make any difference. She needs help, and the kind of help she needs is a professional child therapist to help figure out what is going on in her mind. something important is bothering her that she is not telling anyone, and caning or punishment will not help her...she needs a sympathetic ear she can trust.