Actually, it might be more than just sleeping with Kiki (or all of them to get the bonuses). I think you have to keep them happy, i.e. take them out when they call you. I don't know if you have to get them at 100% liking you, but a lot of the characters will give you call-in bonuses -- Little Jacob gives you discount guns, Packie gives you car bombs, Carmen gives you over-the-phone health, Kiki helps you sometimes with the cops, Alex gives you discounts on clothing, Dwayne gives you two extra guys to help you out, etc.