In the novels, it is unknown if Maege Mormont is still alive (it is implied by her second daughter Alysane that she is), but she actually had three surviving daughters who were older than Lyanna, her youngest. Jon Snow himself wonders why Lyanna Mormont was the one who wrote the reply letter to Stannis (it is possible the Mormonts let her write it to scorn Stannis). Lyanna is apparently the acting "Lady of Bear Island", while her mother and sisters are away, similar to how Bran Stark was the acting "Lord of Winterfell" in Season 2 when his older brother Robb was not physically present to rule.