How do I stop my sister from interfering with my decisions as my Mom's MPOA?

I am MPOA of my mother. My sister confiscated my mothers osteoporosis medicine and her Alzheimer's medicine so I couldn't give them to Mom. I got them back, but now my sister is calling Mom and telling her that when I give the medications to her, put the medications in a drawer instead of taking it.
I heard her ask Mom if I gave her a little whit pill on Sunday morning and did she put it in the drawer. The phone was on speaker phone. I didn't say anything at the time because I didn't want to upset Mom. What can I do to stop this from happening? Can I get a restraining order?