i have alot of bi sexual fren, some of them had gay sex since 14
and etc. its shocking, but its not a minority.
all i can say is that, he is what he is, he sexual preferrance is
in his genes. he can change, but whether he will get happiness is
another thing. as a sister, u should support him no matter how
wrong he is, and not "report" him to your parents and worsen your
relationship. Furthermore, there is nothing your parents can do,
unless they think sending him for counselling or beating him
straight will work. life is like that, its not always straight
just forget about the whole incident and do not avoid your brother
or whatever just because of it. he is your brother after all.
good luck on improving your relation with your brother. who knows,
maybe he wants to be in good terms with you too, just that he never
had the courage. 
