Reunited with my Oxford English Dictionary, I see that it describes this usage of the word "relation", but classifies it as Obs[olete] rare. I've seen this description before, applied to words that are still in dialectical usage in Scotland or elsewhere.

Edit: Oops. Reviewing the illustrative quotations, I see that the "obsolete rare" usage is in the plural: "a numerous relation" meaning "a lot of relatives", for instance. I've certainly never seen or heard that before.

Grant Hutchison