The problem is it isn't really alternative universe you, it's more like an alt-universe brother/sister. Which is weird, but then again my oppinion is that it's the time spent growing up together rather than genetic similarity that makes incest inherently wrong. So as long as protection was used and there was no interaction between our alternative families and we were both willing, it's not necessarily an outright no.
What's with the immediate asusmption we'd be attractive too? I think my alt wouldn't be ugly, but she'd wouldn't be classically attractive, she'd be a big girl, not fat but quite tall and curvey. Though if she looked anything like my sister or mother that would be too weird.
To answer the OP's, I think it's a complicated situation that depends on her personality, that I have no concrete rule on until it actually happens.