I think my mom doesn't deserve the majority of my dad's inheritance
Who do you think deserves it, then?
The money really should go to cancer research
This is something that should have been worked out between your mother and father. It isn't your place to say what should happen.
My dad died. People in his family are all awful and greedy.
I can understand that you do not want your parents' money to go to people who don't really need or deserve it, and your charitable intentions are truly admirable.  However, as the law stands, it is the surviving spouse who automatically receives her husband's property upon death.  You can try to change her mind, but any further decisions are ultimately in the hands of your mother.