Our demat holding company refuses to to transmit account

U do not hv to modify the same. Instead of tht file an additional succession certificate for A. In that you file a specific avertment that you do not hv any knowledge in that aspect what B has committed  and at present i m the only survivor and hence grant me the succession certificate. In any case u can call me on 8149722555

In addition to the Succession certificate, which involves time and money, you can also convert your clarifications as RTI Application and seek it as information.  You will only be spending Rs.22 on postage and Rs.10  as RTI fee.  That not only gets you authentic clarification but also helps in understanding the issue better.  You  can after getting authentic clarification from SEBI as RTI reply, can also issue a notice under CP Act to the Demat company.  

The B  has either intentionally or erroneously added the deceased A into the demat account by forging her signature.

Even though it can be said that B was not mentally alright at that time, this can be termed as an act of fraud or illegal act committed by B.

As a matter of fact B was not mentally alright while opening the demat account hence his application should have been rejected on  two counts, i.e., A being deceased and B being an insane person. 

Now since both are dead, the claim amount or transfer of their respective shares to C, the surviving account holder  is in question.

In fact C cannot claim innocence of A's death and B's mental unstable condition at the time of opening the demat account  hence her suit for declaration and direction to the authorities may also not be entertained or maintainable before district court or the high court.

Therefore as an alternative, C can approach SEBI with an application giving the entire details and seek their intervention  and direction to the concerned broking agency to transfer the shares of the deceased persons to C.

If this step fails to fetch the desired relief, you can take it as a last remedy exhausted by you before approaching court of law for justice 

You can discuss at length with an advocate who is having a good knowledge about the subject matter and proceed as suggested. 

Is this a real time issue? If so, how you are related to the issue or rather, who are the 'we' involved?

Too long a story.

If these are the brief facts, it would be better to consult and engage  a local prudent lawyer for appreciation of facts/ documents, professional advise and necessary proceeding.

thank you so much learned lawyers
much appreciate your varying and valuable perspectives  🙏 
yes in fact I am account holder  B's son
I think ok I on my own one thing I could do is write to SEBI and ask for clarification and RTI may be a good idea
we have already spent a fortune getting succession certificate with name of .A
my father had very strong personality issues . ( vaccination encephalitis more than 50 years ago ---> permanent brain swelling hydrocephalus ---> became worse with passage of time and two episodes of stroke leaving him having strong personality problems and jumbled up memory and the bank officers who came to visit us they khud if they were interested observe it. right now they still claim that my father has done something criminal but fail to understand that insanity can cause people to do the strange things ❗ ❗     )
I really appreciate your replies learned Sirs ⚖️🎩 I will keep you posted with happens
 

It is surprising that how could  a mentally insane person do such things vy cleverly i.e., forging signature of a third person.

Insanity cannot force anyone to  wrong things intentionally and cannot escape the law on the basis of insanity. 

You may wait and watch the developments  and revert if you need any further legal opinion on this subject.