Is zakat due on the sum of money that the sister has not yet received, which is a debt owed to her from the sale of land that was invested in a joint building, and then it was agreed to purchase land for her in installments instead of the debt?

The amount should be zakat-ed for the years of construction and for what remained as a debt before purchasing the land, provided it reaches the nisab and the debtor is willing and able to pay. If the debtor is insolvent, then zakat is paid upon receipt for one year, or for all years, which is more prudent. There is no zakat on the land after its purchase if it is not intended for sale.