Can a will be contested after probate?

My father an uncle had an informal business arrangement. My father preceded my uncle in death and my uncle gave me and my sister $10,000 with no papers. My uncle died 8/18/12, but my sister and I were not informed until 9/21/12 after the will was probated. We rec'd nothing in the will and no financial compensation from the business. Would we be entitled to anything w/o a formal partnership agreement as my father really did help build that business. My cousins are really benefiting financially. from the fruits of his labor. Plus they hid their father's death from me and my sister. What would we have to prove to contest this will. I don't know my uncle's state of mind when he wrote the will or if my cousins may have influenced him.