Sase basically pointed it out quite obviously: if sex would be the "next logical step" after you found you like someone a lot, it would also apply to close family and friends which for most people doesn't apply (plus it's illegal on the family part, at least in my country). Sexual attraction is somewhat different from emotional connection, and while it seems to have some connections on a basic level, it doesn't necessarily correlate. In fact, the correlation for many people at least long-term seems to be a negative one: the more you get to know someone, the less sexual attraction there is.

I believe that the cause of this is mostly because most people I know have only experienced one kind of sexuality which is the adventurous type, meaning you're feeling quite different from your sexual partner and the difference and the difficulty of establishing common ground (with your bodies and your mind) does seem attractive. After you get to know each other better, the adventure aspect fades so most people feel less attracted to each other. There's another type of sexuality that has a lot to do with the feeling of home, of belonging, and this one draws from a feeling of knowing each other well. Usually people are dissatisfied with just the home one (or don't know how to find pleasure in it) so they will seek new adventurous experiences and lose interest in someone they've already "used up" as an adventure partner. It is why good friends will sometimes agree on some sexual exploration with you, it's usually less exciting (since you already know each other which lessens the adventure part) but it can be quite pleasureable anyway if you learn to enjoy the belonging type of sexuality.

As I wrote earlier, you do not "create" attraction, all you can really do is remove the obstacles in your own mind and (if she trusts you enough) help her remove hers.