What is the legitimate Islamic solution for dealing with a Christian mother-in-law who abuses the religion of Islam, when the wife fears for her daughter from her influence, taking into account the mother-in-law's psychological illness and the husband's eagerness to honor her?

Firmness is required in managing the household. This includes observing privacy by not allowing entry into the bedroom except with permission, and ensuring that the house key is only with its occupants. Part of firmness also entails not leaving children alone with a foreign woman if there is a concern about her influence on their religion and morals. If advice does not work with her, the husband must act with firmness and clarity. It is recommended to reward her for her gifts with kindness, connection, and visits to soften her heart.