It's very irresponsible. Had the second dog been the same breed but different parentage there would have been no problem, as it would have been unlikely that similar dangerous patterns of behaviour would be hardwired into it, but the genetic connection makes antisocial tendencies far more likely. It should have been rehomed in a house without children.
Personally I would introduce a puppy of a gentler breed a couple of days before the child came home, as the attack would probably predispose the child to fear all dogs, and that problem should be tackled early.
My son and
his best friend.
Killer dog,owners keeping its brother 
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