My 18 year old sister is having pain in the right lower back area, any idea what could be wrong with her?

Back pain : Low back pain is a very common ailment. If the pain is severe and disabling have her see a doctor.

Many possibilities: It could be musculoskeletal, perhaps related to straining her back somehow. Ibuprofen and rest can sometimes be helpful. Another possibility is it could the manifestations of a urinary tract infection. If she is having any fever or problems with urinating, she should see someone.