Veronica Carrera Chaparro was arrested Sunday, with her mother Angela Chaparro, 42, and sister Daniela Perez, 24.
Police Chief Miguel Ampuero said the mother and sister gave her three options, "sell it, abort it or give it up for adoption."
The child was advertised for sale on Facebook, and the day after the baby was born Nov. 4, a family from Santiago paid formal registration fees of 1,008,562 Chilean pesos ($1,864) to claim it. The buyer also paid Chaparro 60,000 pesos ($111), the Chilean newspaper Cooperativa reported.
At a court hearing Sunday, prosecutor Ernesto Silva called the case "very serious" but admitting it falls within a legal loophole, adding Chilean law does not make it "a crime to offer the delivery of a newborn over the Internet, even though financial compensation is offered."
The charges against Chaparro were unclear, although her mother and sister were charged with aiding in the sale of the baby.
The unidentified buyers were detained and will also appear in court, Cooperativa said.