My baby sister, has power of attorney over my dads affairs.?

She tells him what he is able to do all the time. She limits his spending. He draws a great pension. She had their will changed so everything goes to her. My other sister and I are left out in the cold. I cannot even visit my dad at his house. Everything was to be divided evenly between us until dad changed the will. Now she controls everything. When my mom ped she did not notify me. I found out through a family friend. She also told my dads sisters they are not welcome and they were not to attend my moms funeral. my dad needs his brothers and sisters.He needs their support. He is very miserable there now. She is out of control.I have talked to my dad about this and he is so depressed he just says you know how she is. Is there anything I can do? Is there anything I can legally do to keep her from her from all of my family's things ? She is now trying to get my sister to sign over poa on her. She threatens all the time to throw her out on the street and put dad in a home. She is out of control. What can I do???