Legal Question in Elder Law in California

I don't think there's enough information here to tell. You say the attorney is "overseeing" the trust, but I do not know what that means. Drafting the trust is one thing; administering the trust is another. There are questions about wanting to protect the attorney-client privilege, or the appearance of favoritism of one child over the others.

My typical practice is that if a client wants an advisor in the meeting, and the client knows the risks of waiver of privilege and still wants the advisor, I let the client in. This is not, however, required under the law.

If your mother is competent, it is her choice as to whether to continue with this lawyer as a lawyer. The trust document will say whether your mother has the power to remove this lawyer as a trustee. Your mother may need to meet with another attorney for advice here, if she feels at all uncomfortable with existing counsel.