How to Petition Probate Court to Revise a Testamentary Trust

What is the name of your state (only U.S. law)? Ca

My father created a testamentary trust for my brother (my sisters and I are trustees). My brother never married or had children so we siblings are the next of kin and will inherit his assets if we outlive him. However, the trust created in the 1970s did not stipulate what happens to the assets after all of us (my brother and his sibs) have died. Is it possible for us to petition the probate court to let us add to the trust that each sibling trustee wants to have his/her share go to one of their children if they are not alive?

You'll have to see an attorney. There are too many variables including if the trust has an ascertainable standard. The first attorney I'd go to is the one who made the trust as he may do the work to avoid a potential malpractice suit.

The problem is, the trust may not be entirely your brother's and anything left on his death may escheat.