Are the two boys considered brothers to the girl born to a Muslim husband, and can they live in the same house, after the woman gave birth to two boys from her previous Christian husband?

Lineage is established even with a difference in religion, and a difference in religion does not prevent mahramiyyah (the state of being an unmarriageable kin). So, if the father or brother is Christian, this does not prevent mahramiyyah between him and his daughter or sister, due to the generality of Allah's saying: (And let them not display their adornment except to their husbands, or their fathers, or their husbands' fathers, or their sons, or their husbands' sons, or their brothers, or their brothers' sons, or their sisters' sons...) [An-Nur: 31].

Al-Sarakhsi said: "It is equal whether the mahram is free or enslaved, Muslim or disbeliever; because every person of faith strives to protect his mahārim (unmarriageable kin)." He made an exception for the Magian, stating that a woman should not travel with him or be alone with him, because he believes it permissible to violate her.

Al-Dardir said: "And she may see of her mahram, even if he is a disbeliever, what a man may see of another man, excluding what is between the navel and the knee."