[PW] What's the word for? Co-inlaws or co-grandparents?

[John P. Dyson]

Spanish, Portuguese, Italian and French all share terms for co-mothers 
and co-fathers deriving originally from formal religious sponsorship 
for infants. Compadre-comadre in the first two instances, compare 
(compà, gumbà, depending on where you eat lunch)/comare, 
compère/commère, respectively. As far as I know, none of those groups 
extend term of co-ness beyond immediate parenthood, real or adopted. Of 
course, the words have also evolved to designate friends and chums as 
terms of affection, gatherings of gossipy old biddies and the like. I 
suppose, in a pinch, and for males one might affirm that 
co-grandfathers are (here it comes) beyond compères.

John Dyson