Per Joint Bank Account rules on death, the other joint account holder can operate the account upon the demise of the primary account holder.

2) Most joint bank accounts include automatic rights of survivorship, which means that after one account signer dies, the remaining signer (or signers) retain ownership of the money in the account. The surviving primary account owner can continue using the account, and the money in it, without any interruptions.

3) 

the payment of the balance in the deposit account to the survivor(s)/nominee of a deceased deposit account holder represents a valid discharge of the bank's liability provided:

a. the bank has exercised due care and caution in establishing the identity of the survivor(s) / nominee and the fact of death of the account holder, through appropriate documentary evidence;
b. there is no order from the competent court restraining the bank from making the payment from the account of the deceased; and
c. it has been made clear to the survivor(s) / nominee that he would be receiving the payment from the bank as a trustee of the legal heirs of the deceased depositor, i.e., such payment to him shall not affect the right or claim which any person may have against the survivor(s) / nominee to whom the payment is made.

Let bank to revert and then decide future course. You can get issued a certificate from a gazetted officer to the effect that both names of your sister belong to one person i.e. your sister. 

As the bank account stands in the joint names of your mother and sister who is since deceased, first your mother needs to inform the bank about your sister's death in writing, attaching a copy of her death certificate. The balance in the account shall be paid jointly to the legal heirs of your deceased sister and your mother, and not to your mother alone. The bank may insist on a succession certificate in respect of your deceased sister. Please comply with the bank's procedure regarding settlement of death claims.

You can give application to bank to unfreeze it if not find approach court of law

First of all confirm if the bank has refused to oblige your mother from withdrawing the money held in the joint account.

If it was a either or survivor account, then the bank should not insist on the survivor's request to withdraw the amount.

You follow it up with the bank, if they give in writing refusing to accept your request for the reasons stated therein you may issue a legal notice through a lawyer and demand the requisite act to be done immediately failing which you can either approach banking ombudsman or consumer forum for relief

Dear Client,

First, check if the bank has refused to allow your mother to withdraw money from the joint account. If the account is a "survivorship" account, the bank should not require the survivor to provide any additional documentation to withdraw the funds.

If the bank has refused your mother's request, you should follow up with them to get a written explanation of their decision. If the bank's explanation is not satisfactory, you may want to consider issuing a legal notice through a lawyer. The notice should demand that the bank allow your mother to withdraw the funds immediately. If the bank does not comply with the notice, you may be able to file a complaint with the banking ombudsman or a consumer forum.

You may approach the bank with the death certificate of your sister and the proofs of her earlier name, like SSC certificate, AAdhaar cards etc, and the proof that she had changed the name after marriage. You have to anyhow satisfied the bank that both the names are of your sister who has expired and thus your mother is entitled to operate the bank account. 

- Since, your mother is joint holder in that account , then she can move an application before the said bank to activate that account after submitting her daughters death certificate , and any proof earlier to her marriage  

- Further, if the bank refused then either you can approach the ombudsman or to file a petition before the court. 

- However , your mother  can also approach online to the RBI as well.