Sinemet does not make you "high".... I doubt she is abusing it or that it can be abused in the way you mean. It sounds to me like she's dealing with some hard core stress. My advice is to talk to her about your concerns directly and see how she responds. Caregiving is a hard and sometimes thankless job. It is not unusual for a caregiver to deal with depression, anxiety, or mood changes due to the enormous stress. I would keep trying to convince your mother that she NEEDS to allow support for herself.

If she's "popping pills", they are probably for anxiety (benzodiazapines like Xanax or Valium). Can you talk to her about what you have heard?

Good luck!