What is the ruling on the marriage of my sister's son to his maternal aunt's daughter, given that my mother (the boy's grandmother) breastfed this boy 9 years after her menstrual cycle ceased following her last child, and no milk was coming from her, but rather liquids she called "water"?

The liquid that exits a mother's breast is not milk that spreads حرمة (prohibition of marriage due to fosterage), unless specialists testify that it is milk that has thinned until it became like water and provides nourishment. As for mere water that does not provide nourishment, it does not spread حرمة. The reference in this matter is to medical professionals. If they testify that the liquid is not milk, then it is permissible for the person to marry his maternal aunt's daughter; otherwise, it is not.