Mom is 95 with dementia. My sister is POA but is neglecting her care. What can I do?

Need more information about what you are seeing as neglect.

Not enough details, but at 95 I would place her in AL or MC, where she will have 24/7 care.

More details about what your definition of "neglect" is would be helpful...if you don't have access to see her in person or where she lives I think the only thing you can do is to call APS and report it. But this will further fracture any relationship you have with your sister. You can go to court for guardianship, which costs money and time, but if the judge doesn't like the in-fighting she/he may award it to the county instead.