Brooke Astor's son Anthony Marshall's 1962 divorce decree forces him to pay turncoat sons millions $$$

They testified against their father and could land him in jail for the rest of his life for swindling millions from their grandmother Brooke Astor (they call her “Gagi”), but according to the New York Post, twins Philip and Alec Marshall, 56, sons of Brooke’s son Anthony Marshall, will have a windfall payout regardless of what happens to their father. The rub is that they are star witnesses in the case against him. It is their testimony that could push the 85 year old over the edge and into the slammer.
If pop goes to jail and doesn’t keep the $60 million, they still get one sixth of whatever is left in his estate. If he keeps the $60 million and avoid the slammer, they get one sixth of his estate. That is from the 1962 divorce decree which stated that Marshall would bequeath one sixth of his estate to each son, (one is a photographer and the other is a professor) whom he had with the first of his three wives, Elizabeth Cryan.
Poor Mr. Marshall is said to come from a long history of family dysfunction. Says the AARP:

Belonging to the family of Brooke Russell Kuser Marshall Astor brings unimaginable wealth, power and privilege. It also comes with a debilitating inheritance of alcoholism, domestic abuse, divorce and parental indifference

I wonder why he agreed to give them one sixth of his estate in the first place?  Normally a man like him would pay child support, college expenses and living expenses for the children till they turn 21. But he is not obligated to bequeath anything to them at death. The court can’t force a divorcing person to leave anything to their children in their wills. That does not appear in the Child Support Standards Act or the Domestic Relations Laws as far as I know. It was an option. I wonder why he exercised that particular option rather than just pay their support and then leave them what he wanted in his will?