Pf death claim

As per your situation, you need to have proper documentations which should include the death certificate of your brother, his mother being nominee of the provident fund claim and a letter of authorization which was later on granted to you resulting in making you the next nominee after her death.

These documents along with an application (stating the stages of all the situation/scenarios related to pf) should be submitted to the pf office, after which they may process the requisite amount withing some time (which might take 3 months duration).

The letter given by nominee has no value.  You need succession certificate.