What is the ruling of Islamic Sharia on the dispute that arose between a father and his in-laws due to his son marrying the girl he chose himself, and not the one his father chose?

What happened between your family members is from the instigation of Satan, who incites discord among Muslims, and corrupting relations between people shaves off religion. It is not permissible for a Muslim to abandon his brother for more than three nights, and whoever abandons for more than three nights and then dies, enters the Fire. And whoever abandons his brother for a year is like shedding his blood. All parties must fear Allah and mend their relations, and not sever ties of kinship, for severing them is among the greatest prohibitions. The one who maintains ties is not the one who reciprocates, but rather the one who maintains ties is he who connects his kinship when it is severed. What your brother did by marrying your maternal cousin without your father’s approval, although it is contrary to what is preferable, is not an injustice; rather, your father is the one who erred. You must inform your sister of her true lineage, and her place in her father's house is more befitting than her remaining in her uncle's house, and she must return if her father requests it.