My stepsister is flirting with my boyfriend?

Bless you, you are trying to make light of the situation, but you are clearly very concerned. And I think you are right to be. You need to nip this is the bud right now. Speak to your boyfriend, and judge his reaction. If he displays signs of embarrment or being uncomfortable, then it would appear that things have developed beyond the ten minute aquaintance stage, and he needs to do some serious thinking. You sound a sorted intelligent girl. You don;t want to have to spend your life being vigilant and keeping your boyfriend out of temptations grasp. You want him to be with you because he want to be with you, simple as that. So, cards on the table time.. you;ve been supportive of your step sister, but it sounds like she is over stepping the mark.... he is also straying ever so slightly from the straight and narrow.. if he is seriously considering getting involved with this girl, then you and him need to talk. and, you know what idiots men can sometimes be, a bit of flattery from a pretty girl and they just can;t help themselves.. he maybe just needs a reality hit. But don;t put yourself down, attractive she may be, BUT, she needs to learn about bounderies, sisterhood, and just plain decency.