Oh, I'm CERTAIN it's a test. I've got two good reasons for avoiding this particular bear trap.
1. She (the sister) is there to evaluate me. She'll be judging me, Olympic-style, on charm, looks, wits, and dismounts. As soon as they're safely out of earshot, I'll be the topic of speculative discussion. I know how things work. I'd rather not put myself under the third-party microscope quite so soon.
2. Two girls, be they sisters or friends, plus one male date equals bored date. They'll have tons of things to talk about, and I'll be sitting there playing with my napkin. I've been ignored by girls in pairs before.
So, I'm trying to cautiously reschedule.