Some plans require overage dependent children to be full time students. Depending on the contract this requirement may be waived if the child is disabled. You can find out how your mother's plan works by having her check with her Human Resources Dept.
You should probably contact contact your current insurance provider and ask them to explain your options. If that does not work, I have a few different plans that you make like that i can email you. let me know what they say and I will wait to hear back from you !
adam- the insurance guy