So a line meaning "[even] if it were my own brother the battle would [still] have been tried" appears - possibly as a new addition, possibly as a garbled rendering of a non-brother-related line - and then gets garbled in turn into "if it were not for my own brother the battle would not have been tried". I can believe that - and I prefer it to the idea of poring over the Saumarez family tree in an attempt to identify the original younger brother!