What is the ruling on a sister refusing to give up her share to her brother in a house they inherited from their father, knowing that the father is asking her to give it up and the brother is well-off?

If the father divides his property among his children as inheritance, this division is not valid, and his property remains under his control. However, he is not permitted to favor some of his children in gifting unless there is a legitimate شرعي justification. If the division is a gift and is done equitably, and each child has taken possession of their share, then this gift is valid. If the daughter willingly relinquishes her share in the house for a price less than its market value, out of devotion to her father and kindness to her brother, this is better and she will be rewarded for it. But if she is unwilling, she is not obligated to fulfill her father's request, because it is not permissible for him to take her money to give it to her brother.