Exactly where in the Old Testament does this Jewish law appear? Jesus would not have been bound to follow Jewish custom or tradition unless it is encoded in Jewish Scripture.
There are a lot of possible reasons for why Jesus might have wanted John to care for His mother rather than His brothers. Why Jesus did what He did is pure speculation.
Through critical thinking, you have just arrived at historical truth.
Nice try. Other than Mary being a virgin when she gave birth to Jesus, almost every point you present is highly debatable to say the least.
According to the Mercer Dictionary of the Bible ... "An adult woman in the family was legally considered to be under the authority of the male head of the household - first her father, then her husband after marriage. If her husband died, she could hope to be cared for by her adult children or, if there were none, she could return to her father or be taken in by the next of kin. The Book of Ruth deals with some of these complexities".
Then came the daughters of Zelophehad, the son of Hepher, the son of Gilead, the son of Machir, the son of Manasseh, of the families of Manasseh the son of Joseph: and these are the names of his daughters; Mahlah, Noah, and Hoglah, and Milcah, and Tirzah. And they stood before Moses, and before Eleazar the priest, and before the princes and all the congregation, by the door of the tabernacle of the congregation, saying, Our father died in the wilderness, and he was not in the company of them that gathered themselves together against the Lord in the company of Korah; but died in his own sin, and had no sons. Why should the name of our father be done away from among his family, because he hath no son? Give unto us therefore a possession among the brethren of our father. And Moses brought their cause before the Lord. And the Lord spake unto Moses, saying, The daughters of Zelophehad speak right: thou shalt surely give them a possession of an inheritance among their father's brethren; and thou shalt cause the inheritance of their father to pass unto them. And thou shalt speak unto the children of Israel, saying, If a man die, and have no son, then ye shall cause his inheritance to pass unto his daughter. And if he have no daughter, then ye shall give his inheritance unto his brethren. And if he have no brethren, then ye shall give his inheritance unto his father's brethren. And if his father have no brethren, then ye shall give his inheritance unto his kinsman that is next to him of his family, and he shall possess it: and it shall be unto the children of Israel a statute of judgment, as the Lord commanded Moses.