Green Card for siblings

[venil]

I am a US citizen who had applied for my brother's green card under "family based" green card category.  I got a "request for evidence" letter stating that I am to furnish qualifying relationship document.  I plan to send birth certificates of my brother and I.  There is a glitch though.  The first and last name of my parents in my birth certificate versus that of my brother's - there is a swap.  For example, I got my Dad as Bob Smith, whilst my brother got this as Smith Bob.  My mother is no more.  I wonder if I am to get an affidavit signed by my Dad stating that he is Bob Smith and Smith Bob, and we are his off springs - should this suffice ? or should a lawyer to provide such a document ?  If not for birth certificate what other documents are acceptable to establish siblings relationship.  It is a fact that we are the biological off springs of my parents, but how to prove this other than a birth certificate.