What is the ruling on marriage and living in a shared apartment with one's brother and his wife, such that the bathrooms are shared? And is obedience to the mother, who commands this, obligatory?

If the dwelling has shared facilities, such as a bathroom, then it is not permissible for a couple to live with the husband's brother and his wife, as this exposes them to forbidden khalwa (private seclusion). If one spouse is absent from their partner, the suspicion of forbidden khalwa arises. If the apartment is empty and the brother will live in it after his marriage, then it is possible to live there and then gently and wisely convince the mother that sharing facilities in this manner is contrary to Islamic law. A separate bathroom can be added. What is common practice, regarding married brothers sharing a dwelling where khalwa is not secure, is contrary to Islamic law. Islamic law has warned against being lax about khalwa with the husband's relatives, for "the hamw (in-law) is death."