What is the ruling on the grandson's treatment of his grandfather who does not honor his daughter and abuses her psychologically and financially, and incites the siblings against each other? And what is the ruling on the mother's treatment of her father in this case?

A child must honor and obey their parents, treat them with kindness, and please them by doing what they desire, as long as it is not sinful. Allah Almighty says: "And your Lord has decreed that you shall not worship except Him, and to parents, kindness." The parent's right is not forfeited if they fall short in their duty to their child. Therefore, the child must tolerate what emanates from their parent, seek their forgiveness and pardon, and respond to their mistreatment with kindness. This also applies to grandparents.