Best friend's brother - off limits or go for it?

Anonymous
Long story short, I like my bff's older brother, who is my age. I knw he likes me back, but we don't want to let his sister know at this time - plus, we don't wanna date yet. School and all.

What I want to know is that if we ever were to date, what should we do about his sister/my best friend? Should we ask her first? Do you think she'd get upset? Help.
Best friend's brother - off limits or go for it?
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