It's definitely come kind of incest, since they're still technically siblings even if they are from a different timeline/universe. I think just Incest, since Dynasty's Pseudo-Incest page defines the tag as "Romantic/sexual relationships involving adopted families or step-siblings". They're not adopted or step family, just blood relations across space and time or whatever haha
^ I don't know if it's what you meant, but now I'm imaging GLaDOS being moved from attached to the potato battery to attached to Chell's heart. And that's so disgustingly cute... I love it?
^Well, I read nagatoro's manga and watched the anime, and sincerely I don't remember anything at all happening betweem those 2. I think shippers are just letting their imagination fly a little bit too much here lol