This is a tough issue. Lots of trust money gets wasted on fights between trustees and beneficiaries. I assume your brother is claiming either (a) that your mother was incompetent when she made the change or (b) that she was under your undue influence.
This type of situation cannot be handled online. It will require a face-to-face meeting between you and an expert trusts and estates attorney. You need to schedule an appointment (usually at little or no charge) with such a specialist as soon as possible.
Mr. Huddleston is an Ohio-Certified Specialist in Estate Planning, Trust & Probate Law, with offices in Columbus and Dayton, serving client families and private business owners throughout Ohio. He may be contacted directly by phone toll-free at 888.488.7878 or by email CLH@HUDDLAW.COM. Mr. Huddleston responds to Avvo questions as a public service to help educate and provide general guidance to questioners, but his responses are not legal advice and do not create an attorney-client relationship.
It sounds like a court action has been filed by your brother. You should have been served with the Summons and Complaint. If so, you have a very limited time to respond, usually 28 days. So follow Mr. Huddleston's advice and obtain counsel IMMEDIATELY.
Not enough known facts to work with but I would say that Brother Executor (former co-trustee) may have a real conflict of interest pressing the issue unless he has solid evidence to believe that Mother's change (to you) was not beneficial to the Estate (and its beneficiaries). You need to sit down with a Probate attorney who can review the documents and the factual history to see best way to go. If you believe Brother is wasting estate assets and breaching his fiduciary duties then you can file a Petition to have the Judge issue an order prohibiting him from using estate funds to pursue his vendetta.....this may be a good tactical move to force him to settle or otherwise use his on monies to pursue what he believes to be something that needs to be redressed.
This is not intended to be giving legal advice and this topic can be a complex area where the advice of an attorney should be obtained.