Can my sister with POA prevent me from seeing my mother?

The only way she can keep you from your mom is if she is caring for your mom in her (sister's) home, there is no easy way to gain entrance without her permission.

mkp, we need to have more information.   Such as where is Mom living now?   What are Mom health and memory issues?   Were you her caregiver at one time?   Did you live near Mom in the past?   Why do you think your sister doesn't want you to see your mother?