How is the overdue Zakat from 1996 to be estimated, and is spending on a brother who is a student of knowledge counted from Zakat, and is it permissible to pay the overdue Zakat in installments, knowing that there are current debts?

You must calculate, using the Hijri calendar, the day your wealth reached the nisab (minimum threshold) and the zakat amount, which is 2.5% of the total wealth, including cash and trade goods. This is done at the end of each Hijri year. If you cannot be certain, then make an earnest effort until you are reasonably sure that you have fulfilled your obligation. Paying this debt is immediately obligatory and should not be delayed if you are able. If you are unable, pay what you can, and the remainder becomes a settled debt upon you. The money paid to your brother does not count as zakat due to the absence of the intention (of zakat). A debt settled upon you before zakat becomes due is deducted from your wealth according to the majority of scholars, though it is more cautious not to deduct it. As for a debt settled after zakat becomes due, it is not deducted from it.