“Hey, let’s go on a diet together.” As kids in a small Quebec town, Eisha and Seema were more than sisters, they were soul mates, and a joint diet offered a shared sense of purpose.

Weight-loss jabs such as Ozempic have been a hot topic in Hollywood in recent years, with hoards of celebrities debuting strikingly slimmed-down looks. From the likes of Kathy Bates and Sharon ...