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.