For over 2 weeks my sister (14) has been having cottage cheese - like discharge and claims her butthole has been burning at night. what is happening?

Likely a vaginal/anal yeast infection. It would be best to see a doctor, ideally a gynecologist -- or her pediatrician if they are up to speed on adolescent medicine. However, it probably would be safe to try one of the many over-the-counter vaginal yeast treatments with active ingredient ending in "azole" (clotrimazole, miconazole, etc). For sure she should see a doctor if that doesn't work.

It is very possible that your sister is having a vaginal yeast infection. If this is the first time that she has had one, she should see her primary care doctor or gynecologist for evaluation. There are many ways to treat this, so she needs to get the evaluation soon. Good luck.