Verfication of documents signed in embassy abroad

From your post i couldn't get the details about the Mou you are concerned to.if you have original document signed by your brother and it has a clause binding your brother to do something, then you can file a case for complying with memorandum, either in the country where the subject matter of the claim occurs  or in abu dhabi where the person against whom the case will be filed resides or in the country where the person filling the case resides. 

You can write to Indian Embassy Abhu Dhabi to supply you this information through RTI, but you have to give his name, Passport Number and the likely date of such attestation

Such documents are to be routed through proper channel with specific recommendation of the M.E.A for the purpose of countersugnature by the Embassy/Consulate Office as the case may be.At times both the organisations may be far of.In such circumstances,Notory Action may be of some use...