Ahh vicky...
Dont waste your time and energy in this fruitless exercise....VO's are concerned about Birth certificates of Principal applicant only...let me quote my own example to ease your tension..
I am the principal applicant...and i have a Birth certificate (without name) and my wife doesnt have a birth certificate !
Now, while applying I enclosed just the notarized copy of my birth certificate and class X marks card for my wife (NDVO recognizes this as valid proof of D.O.B- so no worries )
Since I applied, I have received PER from CIO sydney and 2nd AOR from NDVO too, currently status is RBVO and waiting for medicals...So my suggestion is that you dont break your head over petty issues and concentrate on your other important documentation...adding affidavits (how does Indian stamp duty matter to CIO sydney- batao batao??) and explanations just make ur case complex/complicated and confusing for VO....keep it simple buddy !!
And one more thing...if your are really finnicky about birth certificate, you can still have it made after paying a few hundred bucks as penalty for late registration of birth at your local municipal authority's office

Good luck with your application:)
Regards,
JUNER 2011