Affirmation of mahram relationship, even when there is a difference of religion
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.