In France, where wetnursing was an industry in the last century, it is
said that children who were nursed by the same wetnurse felt closer to
her than to their mothers, (naturally), and felt closer to eachother
than to their own siblings, who were often wetnursed by a different
person.  The term "freres de lait" (milk brothers) was commonly used,
without any religious significance.  The term exists in English as
"bosom buddies" as being the closest of friends.  I think the term
exists in Spanish as "hermanos de leche".

Jack Newman, MD, FRCPC