Question Summary:
Can I pay the Zakat due as I am the only one in the family who knows about all his wealth and I am taking care of all his other financial decisions?

Question Detail:

 My father in law has been a strict payer of charity and Zakat all his life. He is over 70 now and I run his business he hasn't paid Zakat for the past 3 years due to being diagnosed with dementia. He asks me the amount he needs to pay quite regularly and then I think forgets about it. Whenever my mother in law asks him about it he says he only has one years to pay. Can I pay the money due as I am the only one in the family who knows about all his wealth and I am taking care of all his other financial decisions including tax, employee pay etc

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
 
May Allah grant you and the family the courage to overcome the health related challenge of your father in law. 
 
If you are deputed by your father in law or known to be his deputy in his financial affairs, you may pay the three years Zakaat due on him.
 
And Allah Ta’āla Knows Best
Mufti Ebrahim Desai.