Is the husband sinful for forsaking his mother and his atheist brother because of their conduct?

The Shari'a commands kindness to parents, even if they are disbelievers. This is particularly emphasized for the mother. A husband's abandonment of his mother is not permissible and is considered an act of disobedience (عقوق) unless there is a legitimate Shari'a benefit for both of them in doing so. As for his atheist brother, there is no obligation to maintain ties with him, and there is no harm in severing ties and abandoning him. Indeed, severing ties might be obligatory if there is a fear of corruption from him. If there is no fear of corruption from him, then it is permissible to maintain ties and show kindness to him, and this might be a cause for his guidance.