How to know if my best friend is ok with me dating his sister?

...ok I broke up with my girlfriend and there's this girl I like were like bestfriends and I want to ask her to winter formal but its by best friends sister how do I know if he's ok with it??? And how would I ask her?

talk to him,be like hey is it ok with you if I take your sister to winter formal,I really like her so do you think it's ok. be nice with him