Talk to him and tell him about your feelings, if he doesnt want you to do it, you have to decide if you're loyal to him or not. If he's a 1-5 star friend, you should betray him. 7+ star friend you should be loyal.
thank me later
bro's before hoes, there will be other cute girls. Friendship > One night stand. Also there is a risk that she doesn't want you and then she's gonna tell his brother and you end up with 0 grills and -1 friend
the funny part is that if your friend is openly against the idea then it means you have already suggested your will to fuck his sister. I can only imagine how the conversation went: "hey dude, I'd do your sister anytime, ya know... just saying... would u be ok 'bout that?"