I would interpret "borderline schizophrenic" to mean that she has some of the symptoms but doesn't quite fulfill all the criteria for schizophrenia. My aunt is "borderline diabetic" which means that her blood sugar is high, but not consistently high enough to be definitively diagnosed with diabetes. I would think the terminology would be similar.

Good luck! It sounds like your sister is lucky to have such a supportive sister!

-apislily