This is unfortunate, but sadly common in families. If your sister is PoA, yes she can keep you from your mom for whatever reason. It would be your burden to prove through the courts that your sister is somehow abusing your mom and for you to have guardianship over her. I feel sorry for your mom. Is there another family member that your sister would allow to visit your mom so that she can be checked on and not suffering neglect or abuse? I don't think you can request a check from APS unless you have sufficient proof that there's something to worry about. Would your sister be open to supervised visits? Just throwing that out as something is better than nothing. If you suspect financial abuse, you would also need to prove this. I'm so sorry you are experiencing this...I wish you all the best in working through or around it.
We had this happen in our family as well. POA would only let very distant relatives visit. We suspected something was being covered up that perhaps a distant relative would not pick up on. Nothing specific that we felt we could report and we feared getting put in the "frivolous/chronic complainer" category. The whole thing was very strange. Any rare visits we were permitted had to be scheduled in advance and were then subject to cancellation on very short notice with little to no explanation. We never questioned whether it's legal - we were afraid to question the POA for fear of retaliation and further restriction of any visiting. It appeared that the distant relatives were not restricted in this way. POA even showed me pictures the distant relatives' visits with elder. Her motivation was never clear, but I strongly suspect there was something POA did not want the close relatives to see or hear.
If your Mother lives with your sister the sister does have a right to keep you from coming to her home. I think in wars of the siblings it is best to decide to get along rather than making the mother torn to pieces in the middle of it all. I would begin by being supportive of the sibling with POA, asking if there is anything you can do to help, by never being disruptive, by never discussing issues with anger or in front of the Mom, but making peace, in other ways. War is never pretty and there is seldom a winner without wounds.
The key is, what does your mother want? Does she tell you she wants to see you? Have you had a consistently good relationship and regular communication up to this point? Did something happen that was upsetting to her? Or is this totally out of the blue? If your sister is misrepresenting your mothers wishes then she is likely overstepping her POA. Have you talked to your sister about it directly? Isolation is a red flag for elder abuse (financial and other) and certainly erodes trust within families. Short of reporting to APS there are sometimes senior care organizations that can be consulted and can offer advice. But don't ignore this - you may end up needing to discuss with APS. You have a right to a relationship with your mother and it isn't OK for POA to interfere (unless you truly pose a threat to your mother's well being).
Good luck.