Ms. da Silva’s 11-year-old daughter became pregnant and had an abortion, prompting a child protection agency visit. After Ms. da Silva told the caseworker that a friend was caring for her Zika baby, the agency investigated and initiated proceedings to remove João Lucas from her home. To keep him from being placed in a shelter, both women and the state agreed that João Lucas would live with Ms. Ribeiro, while Ana Vitória stayed with Ms. da Silva. Under court order, João Lucas spends Sundays at his biological mother’s house.

Ms. Ribeiro, who has adorned João Lucas with a bracelet and necklace hung with a good-luck charm called a “figa,” tries to keep up with his many appointments. They include visits with a psychologist who shows João Lucas a panel of black and white squares to stimulate vision and rubs him with a sponge studded with Popsicle sticks to stimulate touch.

On a visit last fall to Ms. Ribeiro’s emerald green house on a dirt street, where the 23rd psalm hangs on a yellow wall, Ana Vitória toddled around, clutching a piece of spongy cake with one hand, thumping a table with the other. Reaching for her brother’s mouth, she touched the green tape that therapists apply around his lips, fingers, back and chin to relax tight muscles. Ms. da Silva waved a rattle before João Lucas, but he did not respond.

So far, his sister — like the other fraternal twins without obvious brain damage — appears unimpaired, but doctors are monitoring her and the others. At Ana Vitória’s one-year exam, she was slightly behind developmentally. Her vocabulary was limited and she was slow to point to her mother when the doctor asked, Ms. da Silva said.

That could be unrelated to Zika, but, she noted, “The doctor never said it’s 100 percent sure that she doesn’t have a problem.”