We can’t answer your question for certain without seeing your mother's trust, but it sounds like the trust should be changed to have your brother's share of the assets stay in trust for his benefit. Receiving an inheritance may make your brother ineligible for Medicaid or other public benefits that help pay for his care. After your mother passes away, it will probably be too late to put the funds in a trust.
I strongly recommend that your mother meet with the attorney who originally drew up her documents or with another estate or elder law planning attorney. Find a qualified elder law or estate planning attorney near you today.