Conclusion:
These two boys are brothers of this girl through their mother, and they may
live in the same house, because they are siblings, unless there is the fear
of any harm to this Muslim girl in terms of her religious and moral
well-being, from these two Christian brothers or from one of them, in which
case she should not live with them.
But if they are trustworthy and of sound character, then there is nothing
wrong with that.
See also the answer to question no. 21953.
And Allah knows best.