Legal Question in Wills and Trusts in California

Anything can be contested, a will or a trust. If the trust reflects what the settlor wants, she should have a valid "No-Contest" clause in the document and probably leave him enough that it will hurt if he loses.

One further thought -- you might want to have your mother make a videotape expressing her wishes on it. She can hold up a current newspaper (do those still exist?) to show the date the videotape was made. Then put it in a safe place that only you and those you trust will know about in the event your brother tries to mess around with your mother's estate plan.