LENKISEM, Kenya — The first time cutting season came around, Nice Leng’ete and her older sister ran away and hid all night in a tree. The second time, her sister refused to hide.

For Maasai families, the cutting ceremony is a celebration that transforms girls into women and marks daughters as eligible brides. But to 8-year-old Nice, it seemed like a threat: She’d be held down by bigger, stronger women, and her clitoris would be cut. She’d bleed, a lot. Most girls fainted. Some died.

Still, her sister gave in.

“I had tried to tell her, ‘We are running for something that’s worth it,’ ” recalled Ms. Leng’ete, now 27. “But I couldn’t help her.”

Ms. Leng’ete never forgot what her sister suffered, and as she grew up, she was determined to protect other Maasai girls. She started a program that goes village to village, collaborating with elders and girls to create a new rite of passage — without the cutting.