ok first of, why a nun? just take some of your family's riches and go live a normal life, open a restaurant or a farm or something. there's no reason you have to become a nun.
secondly why are you trying to be nice to the whore's daughter? you should still hate her just as much, it's one thing to not bully her but you are actively being nice and calling her sister, what's that about? you're totally ok with your "father" abandoning you for a whore and having a child with her and them all living happily ever after while you wallow in sorrow? this seems incredibly forced.