Zakaat on an investment

I'm the executor of my dads estate. One of the bequests was to pay my younger sister a stipulated accumulative amount on maturity of a fund that's held at Oasis. During this period of maturity which might take a few more years to earn. The amounts accumulated over the last 4 years,do we have to pay zakaat on the current amount, and the amounts over the last 3 years? The account is on my sisters name who is of age, although i'm in control of the account.