Dating Your Friend's Younger Sister?

I am in that situation right now, i am secretly dating my closest friend's younger sister, my friend only had cause to suspect once when he saw his sister's missed calls on my phone, but he didnt confront me; i guess because of the value and respect he has for our agelong friendship, but after that incident when i saw his sister she told me her bros asked her if she had anything to do with me, which she extremely denied, but that didnt stop her brother from warning her that he doesnt want to ever see her with any of his friends, and that if he hears she is dating any of his friends that he is going to kill her,she was visibly agitated and out of fear she wanted us to breakup that minute, but i quickly assured her that i can never allow such to happen , that expectedly calmed her. However in as much as i don't want her hurt; because i have deep feelings for her, i also don't want to have any misunderstanding with my friend; because he is a true friend, Don't tell me am greedy or self-centered, afterall can't i be his friend and be his younger sisters Boyfriend?

Anyway as i speak we are still dating. . . but i can't stop wondering why friends can't just bear seeing you dating  their younger ones, i can't seem to understand it; maybe it's because all my siblings are boys and  so i don't know how it feels. Is it that they think you would "use and dump" her, or their opposition is just instinctive?

Fellow Nairalander, advice, comments are welcome!

did u meet ur friend before the sister?

if the answer is yes, then this is a lazy move.

if u met the girl before becoming friends with her brother, then u have done nothing wrong.

@post

there is nothing wrong and this happen every time but your friendship may be at stake if your friend find out.

how old is his sister (i hope she is not in sec school)

if she is old enough to go into a relationship then there won't really be much problem.