When they were children, Memory Banda and her younger sister were inseparable, just a year apart in age and often mistaken for twins. They shared not only clothes and shoes, but also many of the same dreams and aspirations.

Then, one afternoon in 2009, that close relationship shattered when Ms. Banda’s sister, at age 11, was forced to wed a man in his 30s who had impregnated her.

“She became a different person then,” Ms. Banda recalled. “We never played together anymore because she was now ‘older’ than me. I felt like I lost my best friend.”

Her sister’s pregnancy and forced marriage happened soon after her return from a so-called initiation camp.

In parts of rural Malawi, parents and guardians often send their daughters to these camps when they reach puberty, which Memory’s younger sister hit before she did. The girls stay at the camps for weeks at a time where they learn about motherhood and sex — or, more specifically, how to sexually please a man.

After her sister’s marriage, it dawned on Memory that she would be next, along with many of her peers in the village.