First of all man, sex just changed everything. So if you're both enjoying it, you may as well keep having it. Going from a sexual relationship to a strictly platonic relationship can be extremely difficult, if not impossible due to feelings of either or both parties. You've already crossed that bridge, Beast, now dance on the other side
Second of all, and you'll hear this again: What do you want? Honestly ask yourself what you want from this girl, and make that intention known. If you really want to be just platonic friends, but never see yourself dating her, then insist you two stop having sex and spending as much time alone together. If you just want to be FWB, just tell her that you enjoy her friendship and the sex but have no expectations of a relationship. If you could see her as a possible girlfriend (and you enjoy the friendship and sex it seems, so it's worth considering) then talk to her about it. You can't predict the future, or if you'll develop feelings for this girl, but be willing to bet if you both continue sleeping together you both likely will. It's natural, man.
Third, don't worry about your chances with other women, even her friends. As long as you don't do anything really bad to this girl (lie/cheat/steal/abuse/etc) and treat her well regardless of your status, it'll likely only improve your chances with her friends.
No matter what, it all comes down to communication, actually. Something that took me too long to learn is that you should be able to have an open, honest conversation with whomever you're sleeping with. For now, I would just keep doing what you want, what makes YOU happy, and what YOU have fun doing. If sex with her is one of those things, then keep at it, and let HER bring up where the relationship is going. You can just mentally prepare yourself for that conversation in the meantime.