My other sisters don't get to see our mom because the oldest sister is Power of Attorney. What should I do?

Power of Attorney doesn't give the holder the right to deny visitation. This sounds like a very unpleasant family issue. You may even want to try a family mediator to see if someone can help you all make peace. This is unfair to the parent.

Good luck. You sound like a caring person. If all else fails, an attorney could help, but that is expensive. Counseling, Social Services, or a family mediator may make the POA holder see the light.
Carol