Is it possible for me to sue my step mom for an inheritance?

My Dad passed away three years ago. The day after his death my step mom told me that my father had an ira and that his wishes were for her to get half of and my brother and I were to split the other half. She told me that was what he wanted and she would give us the money as soon as she received the check. Months went by and nothing was given. After several lies and run a rounds from my step mother I found out that my father had signed a paper at his work stating exactly what my step mom had told me. The problem is he signed this paper before they got married and according to the H.R. lady at his work he had to come down and sign another paper stating the same after he married her otherwise all the money would legally go to her, with her getting to do as she pleases with it. I believe she was unaware of this last bit of information and still believed that she had to split the money with us. It wasn't until a few weeks later that she found out that since my father didn't go down and resign papers that she was under no legal obligation to split the ira with us and then she decided that she would like to keep all the money and screw my brother and I out of the money. I would like to know that if her telling me that my fathers wishes were for my brother and I to split the ira with her and she would indeed do that if this forms a verbal contract that she is then obligated to fulfill. She also, after several confrontations, actually sent my brother and half of what we were supposed to get, certified check, and told us that is all we were getting, to bad about the rest. With the verbal agreement and the sending of half of the money does this constitute an admission of "liability" and do I have any real legal recourse as to collect the rest of the money? Is it worth a try?