Massallah pertaining to Zakaah

Assalaamu alaikum Mufti Saab
My question pertains to Zakaah. My mother has some money saved in a Canada Bank account which she opened 24 years ago with the assistance of her brother that lives there. Her brother currently has access to her account. She has requested on numerous occasions that her brother transfer her money to South Africa but he keeps making excuses.
Does she need to pay Zakaah on this money for the past 24 years? If so, how should she calculate the Zakaah for so many years?
Jazaakallah