I'll relate my one little story. There was this HB9 I was FWB with, and had a party with her and this HB7. They both stayed til the very end, til the HB9 got tired and went to my room. The HB7 got the hint and left.
The next night, I had another party and the HB9 wasn't there. While talking to the HB7, I brought it up - "Sorry if you were disappointed last night, but that girl and I are just friends. Well, friends with benefits, but you know." HB7 stayed late again and I f-closed her EASILY. She ended up being a FWB for over 6 months
Letting girls know that attractive women want to fuck you, without even being in a relationship, is a big DHV. Just play it off like it's no big deal. In your case, you can just let her assume you two are fucking, but if she asks then be straight up about it. Now, your new girl knows she has competition, and you're golden as a prized man.