What do you think about me sleeping with my sister's boyfriend?
Why are you going to sleep with your sister's boyfriend?
I want my sister to be hurt
It's very immoral to sleep with someone else's partner, especially someone in your own family.  You should cherish your family and not try to hurt them, you only have one family.  If you betray your sister, she might resent you for a long time and never forgive you.
Why don't you just fuck off?
It is immoral to betray your family and to sleep with someone your sister is involved with. She trusts you and it will impair your relationship going forward.