What is the ruling regarding a mature daughter living with her stepmother, what are the limits of interaction between the children and the stepmother, and is the daughter's husband considered a father-in-law to the stepmother, with a detailed explanation of the rulings on that?

There is no objection to a girl living with her stepmother if she agrees to it. A stepmother is a mahram to her husband's children, so they are permitted to see what a man may see of his mahram (the head, neck, hands, and feet), provided there is no fear of fitna. Seclusion and shaking hands are also permissible under this condition. As for the daughter's wife, she is not a mahram to her husband's daughter's husband; rather, she is a stranger to him.