If there is nothing dubious about the family gatherings, it is permissible for a woman to sit with her husband’s relatives, if she is wearing complete Shar`i Hijab. However, shaking hands with a non-Mahram man is prohibited.

If a man’s stepdaughter – who is the daughter of his wife with whom he has consummated the marriage – has a stepdaughter through breastfeeding, he is a mahram to her, because the mahram relationship that is established through breastfeeding is the same as that which is established through birth.

It is permissible for a wife to shake hands with her father in-law because as soon as a man conducts a marriage contract with a woman, his father becomes a Mahram for her.

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.