The Nomination in equity shares/accounts confers ownership.
Nominee in such accounts is owner.
Other than this Nominee is mere trustee and is under obligation to pass the funds to legal heirs.
The estate of mother (personal and not from estate narrated in WILL of father) can devolve upon her legal heirs as per personal law that applies in her case e.g; Hindu Succession Laws.
You can proceed to act upon the WILL after death of Mother (as per WILL of Father) and from the date existence of WILL came to your knowledge.
If executor is renunciating, you may proceed to submit the WILL for probate.
In any case it is mandatory to probate the WILL in presidential towns that includes; Kolkota.
The estate as mentioned by you is in Kolkota.
Otherwise also: If WILL is contested it WILL land up in probate court of pecuniary jurisdiction.
Here you will require: The trail of funds/estate can be traced back to the days/dates before date of death of father and as on date of death of father and after date of death of father………………………….till current date.