Being an eldest daughter means frequently feeling like you’re not doing enough, like you’re struggling to maintain a veneer of control, like the entire household relies on your diligence. At least, that’s what a contingent of oldest sisters has been saying online. Across social-media platforms, they’ve described the stress of feeling accountable for their family’s happiness, the pressure to succeed, and the impression that they aren’t being cared for in the way they care for others. Some are still teens; others have grown up and left home but still feel over-involved and overextended. As one viral tweet put it, “are u happy or are u the oldest sibling and also a girl”? People have even coined a term for this: “eldest-daughter syndrome.” That “syndrome” does speak to a real social phenomenon, Yang Hu, a professor of global sociology at Lancaster University, in England, told me. In many cultures, oldest siblings as well as daughters of all ages tend to face high expectations from family members—so people playing both parts are especially likely to take on …