Is it okay that I don't feel sorry for my fiance's sister?
Why don't you feel sorry for her?
I hate her, she's even got cancer
You should have sympathy for someone who has cancer. It's rude to not care about your fiance's family.
I don't care. I'm getting married because I plan to have the legal permission to claim on my fiance's life insurance. It's only her and I that matter.
It's wrong to try to trick someone into marrying you for money like that. Your fiancee could learn of your schemes and the marriage will be nullified. The honest thing for you to do is to break off this relationship before you get married, since it seems like you are not marrying her out of love.