Based on your description, I'm assuming your sister has myelomeningocele. I have that form of SB as well, at the L5/S1 level.
Anyway, while reading through your post, it seems a major part of the problem may be stubbornness or apathy. I'd really recommend sitting down and talking with your sister about your concerns. See if there is a reason why she is cathing less than she should (and remind her that cathing insufficiently can cause severe medical consequences).
Also, when you are able to get an appointment with her doctor, ask him or her about ways to improve continence. There are medications to improve bladder health, as Jessie stated, and there are also surgical procedures to strengthen and enlarge the bladder. I had a bladder augmentation, also known as an augmentation cystoplasty, when I was in 3rd grade, and roughly 20 years later, I seldom have leaks (less than 10 times in the 20 years).
I certainly hope you can find out what the problem(s) are and find ways to help your sister. Good luck!