Everything starts with a lady called Rosalind Lutece
Rosalind loves science and particularly quantum physics. She’s extremely clever and she’s one of the best (if not THE best) at what she does!
Her discoveries made a “miracle” happens (“it’s not a miracle! it’s quantum particules!!!”): A city suspended in space-time at a fixed height! a city floating besides the clouds which will never fall!
One day, while measuring her quantum atom, she noticed that it reacted as if someone was answering or was… doing exactly what she was doing at this very moment to be exact.
Which bring you on the topic of alternative dimensions.
She started communicating with the person on the other side of the field, switching the field on and off like a morse code.
It turned out that this clever person who was able to match rosalind’s cleverness was… Rosalind herself. Or not quite! Robert is Rosalind except he comes from an alternative dimension where, instead of being born with chromosomes XX, he was born with chromosomes XY
Same person with a little variant: their gender.
She ended up making him come to her own universe and made people believe they were twins because they wouldn’t believe them (or they are too dumb, pretty much)
He’s her and she’s him, this isn’t a relation you can call siblings, nor twins, nor relatives, nor clones. No “normal” terms can apply because this relation is impossible to start with.
Rosalind is Robert and Robert is Rosalind, they are the same person. (though they do argue a lot which is hilarious to watch)
et voilà! I hope I made it a little clearer!