if you werent friends with him before dont start now because that is excactly what it is going to look like. i'm sorry that you guys have a rocky relationship. my brother and i used to be the same way but since i moved out things got way better. all brother/sister relationships have ups and downs. maybe you should just tell her how you feel and depicked your next actions off of how she takes it. as far as him, your sisters friendship should come way before his. if he is over just say hi and dont sound happy about it or something. or just tell her straight out that you have no interest in him at all cause you have a crush on so and so. but purposly being his friend will make thing really nasty. think of how you would want her to treat your boyfriend and then do that
good luck though 