Sheikh Muhammad al-Manî`î, professor at Umm al-Qurâ University
The guardian for the woman in this case is her son. He is legally obligated to take care of his mother, fulfill her needs, and support her financially for the remainder of her life.
However, cooperation among all her relatives in everything that strengthen ties among them is recommended, and establishing ties of affection are most needed.
Maintaining ties of kinship is one of the greatest deeds by which we can seek nearness to Allah. Likewise, breaking those ties is one of the most serious of major sins.
And Allah knows best.
Source: Islam Today