If you don't have any account documentation, how do you know what kind of account it was? They could have set up any number of different types of trusts that would give them the right to do that. A totten trust could have been established which is basically their money all along - they could withdraw it at any time they please. A legal trust could have been established with clauses that gave them that right. An UTMA shouldn't provide that right. You need legal counsel and advice. We had a lawsuit such as this at a former bank - a trust had been established for the care of minor children. The trustee had a good time with their money. As soon as they were old enough, they sued the trustee. The bank had to turn over all documents and records to the children's attorney.
When you read a great book, you don't escape from life, you plunge deeper into it. Julian Barnes
My Opinion Only.