You probably need to review the Trust to see if the Trustee has discretion to withhold distributions. If you are unsure have legal counsel review the Trust for you.
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Normally an account held under the Uniform Gift to Minors Act ('UGMA') is held & managed by a bank. If mismanagement is claimed, the assertion should be made against the account holder (the bank) with calculations to show how it was mis-managed. Time to have a 'sit-down' with an Estate Planning attorney in Reno / Sparks. See 'Find-A-Lawyer' at the top of this page.
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Generally, a lien can't be filed on trust property without a judgment, however, your question is unclear to me on a couple of points--three questions:
Is your son's UGMA account funded by trust property (or suppose to be)? If so, and IF the trustee has discretion over conflicting claims, then your brother might be able to hold off distributing living trust property. The fact your son intends to file suit against you suggest to me the trustee does not have that discretion, BUT for a clear answer you'd need a trust lawyer to look over the trust document. If you're a named beneficiary and don't have a copy, you are entitled to a copy.
Question #2: Is your son still a minor? If so, he'll need someone to bring suit on his behalf.
Question #3: Are BOTH your parents now deceased? If not, then if one (or both) is living they should also be named trustees and would be able to intervene.
My SUGGESTION is go see a lawyer experience in living trust matters, take a copy of the trust along with you, then get some accurate advice.