Legal Question in Family Law in California

Spouses owe each other fiduciary duties to act in each other's best interest until the marriage is terminated. Once a legal action is filed (divorce/dissolution, annulment, legal separation), there are automatic restraining orders that prevent changing insurance without a written agreement from the other spouse or a court order.

So yes, it's possible to take her off the insurance, but what your brother should do is communicate. Let his wife know that's what he wants to do and seek her agreement. Because your brother can ask that she reimburse these expenses down the road, she has an incentive to cooperate in their moving on from their marriage. But, simply doing it unilaterally exposes him to bad risks.