There doesn't need to be a reason to not like someone. Yes girls can think your great, attractive but not feel sparks. We're funny like that. But on the other hand and I hate to say it... you just might not be that physically appealing to her. And then if you want to get all scientifical about it some say phrenmones do the trick. You might not have the right smell. Who knows. But don't lose your nicesness. Because if your turn into a jerk your only gonna hurt yourself more in the long run. Wouldn't you rather wait for a sweet girl that loved your awesome soul for what it is?
Sleep with her in the first couple of dates. Thats your "brotherly" problem solved. Its that simple. No need to make it complicated.