Sister made father write will to her benefit, what can be done?

Hello Sir, My father wrote a will few years back for the properties he own but within few months my sister convinced him to rewire without my knowledge to her advantage . But, I came to know and one I spoke to my father, he realized the mistake he made and wrote another will, like the first one and fave the originals to me. I sister stays with my parents, so I'm worried that she will try to convince him again to rewrite. What can I do to prevent this from happening? Are there any other alternative like family settlement? My father is willing to do dhana settlement deed but the property is in the bank for a loan. Thanks