There isn’t a fast, simple answer to your question.
However, there are two larger points for where to begin.
First, has her mother been declared legally incompetent? If not, there is a good chance that it is up to her mother whom she sees - or not. Does she want to see her husband?
Second, several states have laws that make it illegal for one family member to restrict access to others from their ailing elderly loved one. It is actually considered a form of elder abuse. So this might be a good place to start. Look into what the laws are in the state where her mother is living regarding this issue.
I’ve heard conflicting stories. Some people say that a POA can restrict who visits the person they have POA for and others say they can’t. Have you asked your stepsister why she won’t let your father visit? Is she refusing everybody the ability to visit or just your dad? Did something happen? Has your stepsister always been like this? I would talk it out with SS and get to the bottom of why. If you don’t get a satisfactory answer, an attorney may need to be your next step.
However, there are two larger points for where to begin.
First, has her mother been declared legally incompetent? If not, there is
a good chance that it is up to her mother whom she sees - or not. Does she want to see her husband?
Second, several states have laws that make it illegal for one family member to restrict access to others from their ailing elderly loved one. It is actually considered a form of elder abuse. So this might be a good place to start. Look into what the laws are in the state where her mother is living regarding this issue.
Good luck.