This is obviously just my theory, so don't quote me on it or anything, but I'd guess the flood is supposed to be bringing in siblings or people that are basically family to us back home. Dick and I don't have any other siblings - [Which at this point is technically accurate, since they don't know Jason's alive, Cass hasn't been adopted yet and no one has any idea Damian's been around this whole time.] So we're each other. I'm not really sure when Dick is from timeline wise, but I'd guess he's closer to me than the me who's usually here, just like how Tim's an older version of Dick than the one who's here normally.

... Basically, the important thing is, we're the only ones we've got. [Also they're pretty crazy codependent and tight even when they do have other siblings, plus because of how things have happened on the Barge, so there's that too.]