The next mystery is The Case of Jean Louis. This one seems to be more of a joke than an actual mystery.
The mystery is that Jean Louis has two last names: Vaurois and d'Imbleval. One name belongs to his mother, the other to his aunt, but he's very shifty when it comes to which name belongs to who. He was recently engaged, but he ran away, saying he has to protect his fiancee from a horrible life.
Again, since this is 1920's France, the woman can't go to Jean Louis directly. She asks Lupin to go to Jean Louis on her behalf and see what is wrong with him.
Jean Louis is at home with two squabbling, older women. He introduces them both as his mother, and tells the long story of his birth. Both women were pregnant widowers, who gave birth around the same time. The doctor had been called out on a case, so an inept midwife was left in charge of the childbirths. She put both babies in the same crib, and completely lost track of which baby was which. That's why Jean Louis has two mothers; nobody knows which one is his real mother. The other baby died that night, so both women laid claim to the living baby.
Jean has had a pretty horrible life, since his two mothers hate each other with a passion. Jean Louis called off his engagement, since he doesn't want to force his fiancee to live with two spiteful mothers-in-law.
Lupin declares that the story is too ridiculous to be true, so he summons the old nurse. After harsh questioning, she admits that Jean Louis belongs to neither woman. Both babies died that night; Jean Louis' father is a stranger with an orphaned child who paid the nurse a lot of money to swap his baby in and keep quiet about it. Everyone is quite happy with this resolution, especially the two mothers, because they don't have to live together anymore.
On the way home, Lupin admits that he learned about the story ahead of time from some locals, since it makes for amusing gossip. He bribed the nurse, into making up the story about the orphan child, as he was fairly confident the mothers would accept any excuse to be rid of each other forever. He was right.
This was a fun story, with the two older women fighting a lot and the overly dramatic tale of the accidental baby mixup.