My brother emailed me stating that our father died mysteriously due to a blow in the head

Assuming that your father did not die mysteriously from a blow to the head and you did not find your father's body, you might, to the extent that your brother was implying to others (it doesn't matter what he said to you) that you may have had something to do with your faither's death, have a claim for defamation.  However, statutes of limitations for defamation are generally short (1 year in NY, don't know about LA), and it sounds as if most of this took place quite a while ago.