Pierre et Jean is not a short story but a novel about the two very different brothers of the title, Pierre recently having qualified as a medical doctor (after some repeat student years) and Jean having just qualified as a lawyer. Both have shown some interest (if apparently only casually, maybe not seriously) in the young widow Mme Rosémilly, who has only seemed to show interest in Jean.
Initially this novel appears to concern Pierre's jealousy: Léon Maréchal, an old friend of the family, has died and leaves quite a fortune to Jean, but nothing to Pierre. Not unnaturally Pierre resents this, but resents it even more when he suspects, on information a female bar server acquaintance, that Maréchal may have been Jean's real father, which throws him into even more depression because she's perfectly right.
So Jean's only Pierre's half brother, his mother had Maréchal as a lover, and Pierre goes off as a doctor on a ship, leaving his mother with her guilt, his father in ignorance of the whole business, and Jean in his wealth with his new wife, the widow Rosémilly.
My other Guy de Maupassant post:
Maupassant: Contes de la bécasse | Tales of the Woodcock