Not only Satan does these things- but all his myriads of demons scattered throughout the Earth stirring up anger and resentment as well. They feed off of our negativity like a parasite with refined Sugar we eat.
She is no doubt justified in her resentment towards her your father- who , Im sure allowed Satan or one of his demons to use him to cause hurt towards her- causing this cancer.
Nevertheless-- Jesus gets the last word in the matter- and we are all held responsible for what we did with our lives. We have to forgive others, yet be assured that the person who we have to forgive is still going to have to answer before God for their actions wether that person they hurt prays for them or not.
Her forgiving him will only help her soul all the more and free it from bondage but as far as your father's soul is concerned, he still has to answer for what he did and will not get away with his actions before God.
Man was not intended to be made for the purpose of allowing Satan to use us-- but we often fall into this trap many times- and hurt others because of it- scarring them for life. We were created to be vessels for God- -but we have freedom of choice- which we abuse and use against God instead of choosing to live FOR God.
What your sister can do is say this prayer,
"Father, I dont have it in me to forgive my Dad- so may I ask you to forgive him Through me-- I dont have the ability, God but I know you can give me Your forgiveness for him"
If she doenst feel like saying this prayer, she can first pray for God to put the desire to forgive her father in heart first.
Tell your sister not to worry- your father still has to answer for his actions whether she forgives him or not- it isnt going to be easy for him before God.