> þar sem hann hefir drepið Njál föðurbróðir minn (maybe Modern spelling of accusative?)

I think `föðurbróðir' must be a typo. The accusative is demanded by the context (compare `Njál' and `sonu', accusative as we'd expect), so it ought to be `föðurbróður', in both Old or Modern Icelandic. Konráð Gíslason and Eiríkur Jónsson's edition has the correct accusative form `föðurbróður' [ http://dp.rastko.net/projects/projectID438a3f4017f9f/projectID438a3f4017f9f_TEI.txt ].