My dad is in age care my sibling has been controlling everything. She previous lived with him. Unfortunately sibling and myself have been estranged for some time I don't think she would contact me if something happened to dad. As she holds the power of attorney and nursing home by law is under no obligation to inform me. As a family member is my sibling required to contact me.
We are only getting your side of the story so maybe there's a reason she is blocking you.
Maybe offer to agree to supervised visits with your Dad? It's better than nothing.
You could just go to the nursing home to visit your dad. Unless your sister has blocked you from visiting, you should be able to visit, depending on what the rules and procedures are. The staff may need to ask your dad whether it's okay for you to come in and see him.