My dad had expired recently. My mother was his second wife. He married my mom after almost 5yrs the

My dad had expired recently. My mother was his second wife. He married my mom after almost 5yrs the death of his first wife.My father kept my mom as nominee in all his official benefits including GPF, GL etc. But still agreed to give 1/3 part to dad's first wife's daughter who's married with two children (boys). Now the girl and her husband are harassing us and sent a letter to officials and MRO saying that my step mother and step sister are depriving me from taking those amounts, so add me as family member and give me those amounts. We are shocked by this letter. We want justice legally sir, we don't want any illegal amounts. We are in a bad financial situation and totally dependent on my dad. Please help us what benefits we will get legally.