it's the way that ruben's abuse towards everyone else is pure rage and violence, but with niall it is always the abuse of a father. punishing niall like he's a disobedient child: clean the house, mow the lawn, show me a picture when you're done so i know you've done it well. i pay for everything you have.
on top of the emasculation and on top of the way he resorts to sexual violence every single time. it's the way he calls him son as much as he calls him brother.
AND the worst part of it is that it's the making of that bond that creates the vehicle for the violence! ruben insists that niall is family (they both do) and so he's allowed to hurt niall in the way that he learned family is allowed to hurt each other. he does love niall, and so he has to love him in the same way that he learned to be loved.