A Legal heir is entitled to inheritence of the properties (both movable and immovable) left behind by a person (without making a WILL, or without in any way disposing of the property - either by way of GIFT or sale etc.)
A nominee on the other hand is only a Trustee for obtaining the money lying in the banks/financial institutions. The nominee receives and holds the amounts in trust for the benefit of and distribution to the legal heirs. Nominee cannot appropriate the amounts to himself/herself. In case the nominee happens to be one of the legal heirs, then in that case he/she can take away only is/her share and not the entire amount.
In your case, if there is nomination, then the Bank/Financial Institution will honour the nomination and release the amounts to the nominee. But to prevent the same from happening, you have to approach a civil court for restraining either the bank from releasing or the nominee from appropriating the money. For this, you have to approach a lawyer in your locality, discuss the matter with him and take appropriate legal steps without wasting time. This is necessary because, once the nominee takes out the money and utilises it, then it will become a very protracted battle to recover your share from the nominee.