Is my mother-in-law entitled to disinherit my husband from his father's inheritance, claiming that he married during his father's lifetime, and to force him to relinquish his share to his brother? And is this considered haram, thereby forfeiting the right of my young children to the inheritance?

A mother's deprivation of her son from inheritance by compelling him to make a power of attorney for his brother so that he takes nothing from his inheritance is considered a grave sin, because it is an transgression of the limits set by Allah. Similarly, it is not permissible for her to burden him with expenses and demand what harms him, due to the Prophet's (peace be upon him) saying: "There should be no harm nor reciprocating harm." Your husband should seek the help of those his mother respects to make her understand that depriving her son of inheritance or burdening him with expenses is not permissible, and it is a severance of kinship ties.