To really know that you have been disinherited from this trust you have to see a copy. You need to look at the beneficiary of the trust during your mother's life (likely your mother) and after her death (likely you and your brother). If this is the case then the trust is likely something good that protects your inheritance.
If the trust does remove you as a future beneficiary after your mother's death then the way that you prevent that from happening is to prove that your mother was "unduly influenced" into executing the trust. It is very difficult to prove undue influence. You are going to have to show that your mother was forced into executing the trust (like a gun to her head). Gun to the head type forcing is easy to prove, the sort of undue influence you will have to prove is much harder to prove like intimidation or fear of being alone. Most of the time to prove undue influence you have to show some amount of mental incompetency as well.
If you can prove undue influence you must file suit against the trust and the trustee of the trust to request a court overturn the trust. You will possibly have to do this before your mother's death.
I cannot provide you with legal advise. I have provided you with information about the law related to your question. My answer, and any information that you find online, should not take the place of having a consultation with a lawyer in your area to advise you regarding your specific issues.
Please show your appreciation for my candid, accurate information by clicking VERY INFORMATIVE, OR the OR and positive feedback. You should only rate me if you are satisfied with the information I provided you. If you cannot rate me GREAT please do not rate. Give me a chance to make it right by clicking REPLY TO EXPERT.
I appreciate your question and repeat customers. You can request me by beginning your question with "Dear XXXXX..."