Must I wash my father's clothes by hand, despite the pain this causes me, and would I be sinful if I do not?

If your father is in the state described, he is in great danger, but his right to your dutifulness and good companionship is not forfeited.

If he commands you to do something permissible, for a valid reason, and without harm to you, then it is obligatory to obey him, even if it involves hardship for you.

If washing your father’s clothes by hand causes you harm, then you are not blameworthy for refusing. However, if you are not harmed, then it is obligatory to obey him, even if it involves hardship.

You must be dutiful to your father and not mistreat him. Among the greatest acts of dutifulness to him are advising him, enjoining good upon him, and forbidding evil to him with politeness and gentleness, and striving to help him repent and become righteous.

Ibn Taymiyyah said: "A person is obligated to obey his parents in matters that are not disobedience [to Allah], even if they are disobedient [to Allah]... This applies to matters that benefit them and do not cause harm. If it is difficult for him, but does not harm him, then it is obligatory; otherwise, it is not."