I've realized there's no English term for one's child's in-laws. There should be one, for we share a bond of love for our wedded children. So I think I'll just call Maggie my "co-mother".
I am so happy that she and Merv live close to Rhea and Tony, and that they, too, are delighted with the choice their child has made. I couldn't ask for a nicer family to be part of. It will make the parting a tiny bit easier.