You put a calming hand on your girlfriend's shoulder and explain, "Jill, this is for Adriana. Not me." You look her square in the eyes, as serious as can be. "I'm doing this for her pleasure." She shakes her head and huffs in exasperation, "Your chivalry is sweet, but are you fucking crazy? I told you before, if you don't know who you are, you'll be lost!" It looks like you might be gaining the upper hand here, as she's less interested in getting naked with her stepsister and more invested in mentoring your weak will. Too bad for you there's a pretty clear bulge growing in your jeans as the two girls continue to strip and your girlfriend pauses.