RTI in getting FIR and Post mortem report

Yes, it can be obtained through an application under section 6 of the Right to Information Act, 2005.

It would be better to consult a local prudent lawyer for appreciation of facts/ documents, professional guidance and necessary action.

I am not aware that the FIR and PM Report is required for Death Certificate when it is a suicide.

All FIRs must be uploaded in website.  You can file RTI Application seeking certified copies of FIR and also post mortem report.   Apply to Hospital also.   

Yes, the said documents could be sought under RTI. But, to my knowledge, it could be incorrect understanding that those documents are prerequisite in obtaining death certificate. Please verify.

Even otherwise, the certificate could be accessed through the website as well.

There is a column for Reason for death, which has to be recorded.  Normally authorities write the reason as stated by applicants but in MLC case they insist for such reason through the document (Claim for insurance in case of suicide etc.,or suicide converted as natural death for claiming insurance etc)

since your brother committed suicide and died, you can get the copies of FIR and PM report from the concerned police station.