In the given circumstances, presuming you are Hindu by religion, if your mother is not alive then the assets left behind by your unmarried brother shall devolve on his class II legal heirs upon his intestate death.
Section 8 of the Act lays down the general rules of succession in the case of Hindu males who have died intestate and inter-alia states that the property of such a male Hindu shall devolve firstly, upon the heirs of the deceased, being class I heirs.
And secondly, if there are no class I heirs, then upon the class II heirs.
In your case, since your brother did not have any class I heirs who survived him at the time of his death and if your father was also not alive at that time, then his brothers and sisters (assuming he doesn’t have step-brothers or step-sisters) will inherit his estate simultaneously and in equal shares.
For this he has to file a case before appropriate court seeking succession certificate because the revenue department is not authorised to furnish Class II legal heirship certificate.
Since one of your brothers is trying to usurp the property illegally, you may have to write to bank or post offices where ever your brother has made deposits by sending a legal notice stating that so and so died on so and so date, however one of his brothers is trying to acquire the assets lying on the name of the deceased account holder illegally by deceiving the other successors in interest they may be instructed to not to disburse the claim amount until a succession certificate is produced by all the successors in interest, simultaneously you may have to file an application restraining the bank and post office from disbursing the assets o anyone till the disposal of the succession petition pending before the court.
In all the probabilities, you cannot have a relief outwith approaching court of law until and unless the agitating brother comes in compromise terms with other legal heirs.
So don't waste time by investigating ways to avoid court case because by that time your brother would have drained out all the money after which it will become very difficult to retrieve your share of money from him by any means including legal action through court of law. .