The money in the UTMA 529 account belongs to the beneficiary, in the legal sense. The only person who can choose to transfer the money to a sibling (or any other relative under the 529 rules) is the beneficiary (who is also the legal account owner), and that decision should not be made under duress or coercion, or while the beneficiary/account owner is a minor.
That's interesting that the financial institution set up the 529 accounts as UTMAs, because that is certainly not the default course of action.