Nope. First, ask for his blessings. Let him know that you do not mean any harm on his sister. And make sure that you wont break her heart or lead her on something that will never be. Worse thing that could happen is you break his sister's heart and then you lose your friendship with you bestfriend. Dig? There aint nothin wrong to it, just make the necessary precautions in order for everything to glide smoothly. Alright? Goodluck 5-0! Keep up posted on any developments.
jk loverboy! haha Again, Goodluck buddy!