What makes you feel that your your Dad has not taken care of by providing for your Mom and and dividing assets equally at her death. If your share is enough to help you considering you are receiving SSDI then your Dad has done exactly what he said. Don't envy the success of your siblings and turn this into a family fight.
There are absolutely different ways to address your situation depending on what your dad's objectives were. As my colleague has stated, it may be that his objectives were absolutely covered by the plan that you describe. That being said, you can discuss this with your mother and also consult with a local estate planning attorney with all your information to see how all this impacts your situation and what if anything your mother might want to do if dad's objectives were other than what was prepared. Good luck.
I would meet with an estate planning attorney to fully review the situation. You should be prepared that simply because your father assured you verbally that you would be "taken care of" does not mean that you can now enforce that promise. However, as you pointed out, there may be some options that are available to your family. Nonetheless, any trust your father had created during his life would have become irrevocable at his death.
It sounds as if you are receiving SSDI, but may also be receiving means-tested government benefits. Is that the case? If so, the influx of cash to you from whatever inheritance you received from your father's estate could be enough to effect your eligibility for those programs, including SSI, Medicaid, and Medically-Needy - all of which are means-tested. If maintaining eligibility for government assistance programs is meaningful to you, you need to consult with an attorney who is very familiar with Special Needs Trusts to determine how best to handle the assets of the current estate so that your benefits are not put at risk.
My answer is of a general nature and should not be construed to be legal advice nor creating an attorney-client relationship. Carol Johnson Law Firm, P.A. practices in the area of Wills, Trusts, and Estates, Disability - with a particular focus on providing Special Needs Trusts for disabled children and adults.