Praise be to Allah
Lineage is affirmed even when there is a difference of religion, and difference of religion does not cancel out the mahram relationship. If the father or brother is Christian, that does not cancel out the mahram relationship between him and his daughter or sister, whether the sister is a full sister or a half sister through the father or mother, because of the general meaning of the verse in which Allah, may He be exalted, says (interpretation of the meaning):
“… and not expose their adornment except to their husbands, their fathers, their husbands’ fathers, their sons, their husbands’ sons, their brothers, their brothers’ sons, their sisters’ sons…”
As-Sarkhasi (may Allah have mercy on him) said:
It makes no difference whether the mahram is a free man or a slave, Muslim or otherwise, because every follower of a religion will respect the boundaries around his mahrams, unless he is a Magian; in that case she should not go out with him, because he believes that she is permissible for him, so he will not cease to think of inappropriate conduct with her. Therefore she should not travel with him or be alone with him.
End quote from al-Mabsoot (4/111).
Ad-Dardeer said in ash-Sharh al-Kabeer (1/215):
And she may see of a mahram, even if he is a disbeliever, that which any man like him could see, apart from the area between the navel and the knee. End quote.
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.
And Allah knows best.