In my fic universe, that is. No, it's not a reference to their Irish ancestry. Apparently, children that are less than twelve months apart in age are called Irish Twins. Marlene is born on September 5 of 1997, while Marty Jr is born on June 17 of 1998.

I'll probably have Alexander Patrick McFly and Michael Jandrew McFly in the Almanac Universe be born on the same day as Alexander Jandrew McFly in the Twins Universe - and just a few days apart from Marty Jr. I'll probably also have Alex from the Almanac Universe be the interdimensional counterpart of Alex from the Twin Universe - even though their fathers aren't the same, and neither are their mothers.

Marty III is the same person in all the universe - even though his mother is Harmony Brown in the Trilogy and Haven Universes, and Stephanie Stebbs in the rest.

In the Twins Universe, Calvin marries April Schwartz - while, in the Almanac Universe, Calvin (Hell Valley Marty) marries Sarah Parker (Jennifer's cousin).

So Irish twins are the closest two siblings can be to being twins without actually being twins?

Pretty much, yeah. Pregnancy generally takes nine months, but some people are born early. I do wonder how closest a pair of biological siblings has been in history, without being twins.

I think two half-siblings can be born the same day, those with the same dad, but different moms.

That true. I have Tiff Tannen a little younger than Jennifer Parker (the older one), but they're probably closer than nine months apart. So, in the father's side, they can be close in age - but not the mother's side.

Try this for an interesting concept: half-identical twins who have each received their sperm from different father. Probably very, very rare - but I think it could happen. Of course, that would also mean the mother is most likely extremely promiscuous.

Twins with different fathers sharing an egg? That probably could happen.

That's be kind of cool, actually - although it might not speak very well for the mother.

Here's how I figured the Quagmires became triplets. Duncan and Quigley shared a zygote, and then it split. Isadora, on the other hand, had a separate zygote. So Duncan and Quigley are identical, while Isadora isn't.

BTW, how similar in appearances do you think Michael J Fox and Lea Thompson are? They do both have the same hair colour.

They're also both the same height. Marty was sitting next to Lorraine when Stella told him he looked familiar.

Maybe I should change the height of the girls in my fics, including Harmony. On the plus side, it would now make Harmony be the same height as Michael J Fox. I just hope it won't be too much hassle, as I have quite a few fics to go through. I'll maybe do that later this week.