Is it obligatory to obey the father by bringing the disobedient grandson back home, and does the sister have the right to prioritize caring for her disobedient son over caring for her elderly father, taking into account that the father needs his daughter to care for him?

You have all erred. The one most in error is your nephew, for striking his father and being undutiful to him. Your sister's error lies in abandoning her father because you expelled her son. As for your error, it is in expelling your nephew from the house, and you have incurred sin. You must bring your nephew back, in obedience to your father and out of kindness to your sister. This is better for him than surrendering to a life of crime.