Is it permissible to stand up to one's father and defend one's mother, whom her husband accuses of being "children of haram," knowing that she is over sixty years old and he performs Hajj frequently and is常 present at the mosque?

Your father's treatment of your mother is forbidden, and he must desist from it and repent to Allah, for Allah Almighty says: ﴿And those who harm believing men and believing women for [something] other than what they have earned have certainly borne slander and manifest sin.﴾, and the Prophet, peace and blessings be upon him, said: "Every Muslim's blood, wealth, and honor are sacred to another Muslim," and "Treat women kindly." And his saying "You are illegitimate children" is an accusation of adultery against the wife, which is one of the gravest sins, for the Prophet, peace and blessings be upon him, said: "Avoid the seven destructive sins," and he mentioned among them "slandering chaste, believing, unsuspecting women." He must offer sincere repentance, fear Allah, and treat his wife well, knowing that he will stand before Allah. In the Hadith: "Whoever says about a believer something that is not true about him, Allah will make him dwell in the mire of Khabal because of what he said." As for the questioner, he must advise his father and remind him of Allah, and defend his mother's honor with kindness, without insulting him.