Is this partial distribution by the trustee legal?

The trustee can make the distribution anyway; he's asking for the Waiver so he knows he won't get sued over it trying to keep the trust's expenses down. If you sign the waiver, you're saying you won't sue over the distribution. So, if you don't sign the waiver, then the trustee has to assume you're going to sue, and so decides not to do the thing you plan to sue over. We often want to do a partial distribution before the whole trust has been settled. It is often helpful for tax purposes to distribute to the beneficiaries each tax year, and there is no sense in the trust hanging on to more cash than it needs to do its business. At the end of the day, the trustee has to do what the trust document says he must do. However, the exact steps taking to reach that result are within the trustee's discretion. If it says the trust is to distribute equally to four people, then that's the end result. If three people get money now, and the troublemaker waits until the very end to get his, this just creates more accounting work. Finally, the attorney represents the trustee; if any beneficiary wants to be represented by his own counsel, he should hire counsel and follow the advice he gets.