Even if they are likely not blod-related I still think that there is a true bond, at least close to adopted siblings, between Page One and Ulti given how protective she is of him and her nickname of him.
It doesn't appear to be stated outright, but it is pretty heavily suggested though.
Sasaki insults Page One:
Ulti quickly rises to his defense and tells him that "she'll tear him apart":
Page One tells her to knock it off:
Who's who calls them brats and tells them to shut up.
Their banter seems like typical sibling banter to me, and the way they are called "brats" together is suggestive I think. Perhaps the siblinghood is implied in the raws. We'll see when better translations come out I guess.