Does a trust beneficiary have a right to know how much extra money a sibling beneficiary received to administer the trust?

You can file an action against the trustee or successor trustee who is administering the trust, for an accounting, and he will have to produce all the records that show all the disbursements to each beneficiary and to each creditor or taxing authority if any. You should also read the terms of the trust to determine if he is required by those documents to file an annual or semiannual accounting or other periodic accounting.