How can one deal with a father who walks around the house in clothes above the knee? Is washing and hanging his clothes considered assisting him in sin? What is the ruling on not advising him beforehand?

The majority of scholars hold the view that a man's 'awrah (private parts that must be covered) is from the knee to the navel, while some opine that the thigh is not part of the 'awrah. If the father believes that exposing the thigh is forbidden, he should be gently and politely advised. However, if he believes it is permissible, then there is no need to object to him. It is permissible to wash short clothes that reveal the thigh. One must lower the gaze from the thigh if the questioner believes it to be 'awrah, and this should be done to the best of one's ability, while a sudden glance is excused.