Adoption Puzzle

Here's a puzzling situation I'd like to present to y'all for your ideas. I am researching a guy who "according to family stories, was adopted by his wife's parents." Huh? This apparently happened when he was an adult. Odder still, he himself adopted a girl quite late in life, whose parentage was "unknown". Even odder, her name appears to change from Minnie to Mary, unless those were 2 different individuals. At any rate, that strange story of his adoption has the ring of a strange truth...but why on earth would his inlaws do this? Sweetie says something about an inheritance...but--they could just give him stuff, right? Why would he need to be adopted? Bit flummoxed here....