If you have a tendency to believe liars, then I might think that you were repeating that pattern now. If new friends can't tell you and your twin apart, why wouldn't this guy? I am assuming you and your sister have different last names? Are you not connected on Facebook or anything (even just through a shared connection with a parent or another relative)?

There are other guys out there. You are not that invested in this one yet if you have just been on a few dates. I'd lean towards cutting my losses now. His story just doesn't seem believable to me.

IF he only went on a date with your sister and never spent the night, I could see letting this slide. If things went any further than that, I would lose interest.

It really is a tough choice you have to make!