Why is 'son' the only immediate family descriptor that doesn't end in "-er"?

by unpaidinvoices
OldWorldGlory

You might want to crosspost this to /r/etymology for specialists to look through.

Searocksandtrees

This submission has been removed, as it's not a history question. Consider x-posting to a language sub, eg one specializing in English, or /r/asklinguistics (in their stickied Q&A post), or, as suggested /r/etymology