Tell your best friend your dating his sister
If she is interested then you need to discuss the brother.
Once you're both happy to tell him, go out with him on your own and let him know you want to date his sister.
If there is no chemistry between the two of you, and you have never even talked to her, perhaps you should get to know her first before you ask her out on a date, so as to keep yourself from an awkward situation if she were to turn you down. Reassure her that you have talked to her sibling first, and that you just want to get to know her better on your own.In that case, I can see problems arising indeed, but maybe the friendship isn't all that important.I've never really been in your shoes, but I can give you something like your brother's perspective: Are you an asshole? It takes some reflection to realize you are one, and sometimes your friends won't tell you straight up about it.What I'm getting at is: Has your friend seen you treating other women with respect?If so, I don't think there would be a problem if you're honest with him.