What can we do as stepchildren to reassure my mother’s wishes are granted?

My mother and stepfather were married for 28 years. My mom had a holographic will. She passed 14 years ago. My stepfather passed recently. They owned a home. We were not adopted by my stepfather. He had no will. In my mother’s will, she requested my sister to oversee a certain bank account to be equally distributed amongst her children. My stepfather took it among himself instead of my sister to oversee that and he only gave us $10,000.00 each. There was an overabundance of money in that account that he kept the rest for himself. Now, that he has passed, his half-brother has taken over everything including what is left in that bank account. It seems like us, children, are not going to get much of anything. My mother was the breadwinner in that relationship and paid for most everything. My brother had my mother’s will all this time and just recently told us other 3 children about it. My sister and I are on SSI and my brothers are financially stable, however, my brothers are not willing to help my sister and I out in this matter.