They are engaged, which leads me to believe that they should each other well enough to know the difference between the twins. So still no excuse.

As I said, even if it's stupid... We don't know how long they've known each other or how long they've been together or if he have met her sister a lot of times or how alike they are. She might be great at impersonating her sister. And she did had sex with hers fiancé. But yeah, he's probably just full of shit and had sex with her twin and used a lame excuse. They're both assholes though.

Not to mention that he could have been in an altered mental state- drunk, pot, sleep deprived... There was a set of twin girls at my middle school (YES, MIDDLE SCHOOL, and 6th grade at that!), and every time the 'nicer' (...comparatively) sister got a boyfriend or girlfriend, the other sister would do any and everything she could think of to trick them into having sex with her. Methods that she was confirmed to have used included spiking sodas with alcohol and/or sedatives. Of course, the sister they were actually dating preferred to wait until marriage, so every time she did it it led to very public blowup a between the two of them. That said, that could have just been my hometowns specific brand of crazy, but if OP's twin is anywhere near as odd as that girl was, I wouldn't be too surprised if she had at least taken advantage of him being under the influence, even if she didn't actively push him to drink. Either way, I would personally put a big hold on the relationship until things were straightened out, and then question if I wanted to be in a relationship with someone who might have a repeat scenario occur.

I remember a FML from before when the groom was worried that identical twin sisters would switch at the wedding to play a joke on him because they are so alike in personality and pretended to be each other before.