I think you might need to relax a little bit around gay issues, frankly. People might make all sorts of silly assumptions and you can’t feel responsible for them. Who cares if some random person you’ll never see again thought you might be a lesbian?
As long as you weren’t doing anything intentionally to give them that impression (which, presumably you weren’t…I’m hoping you weren’t cuddling or kissing your sister…:)) then you’re not responsible for any dumb notions they might have come up with.
Even when the person asked, “Oh, is this your girlfriend?” or “How long have you been dating?” you don’t need to go into a tailspin or freak out. Just laugh and go “No, I’m straight. This is my sister.”