If they wanted to control what she did with the funds they should have set up a trust. That letter is meaningless.
As per Indian Law, the custodian is authorised to do anything for the exclusive benefit of the minor.
My parents took out a mutual fund for my daughters college expenses when she was younger. They stated on a notarized letter (which they can't find but redid it recently) that this is to be used to assist in my portion of her college tuition (I'm responsible for 50% and her mom is responsible for the other 50%). My ex is claiming I can not do this since it was taken for her, but it was my parents and they stated it is for college (I have receipts for everything paid)
It is a custodial account which I am the custodian and she is the minor it's a UGA