What is the ruling on hitting a younger sister who misbehaves and disrespects her elder siblings, despite the mild reprimand of the parents, and how can one deal with her and with the parents who condone her actions?

Islam has created in this what is authentically reported from the Prophet, peace and blessings be upon him: "He is not of us who does not respect our elders, have mercy on our young, and acknowledge the right of our scholars." Therefore, one should deal with a younger sister with mercy, gentleness, and kind words so that she learns good manners and respects elders. Beating, in most cases, is not beneficial. If it occurs, it should be with the permission of the parents, in the narrowest scope, and light, for discipline, not for torment.