Maybe they meant that they were the only surviving female members of the tribe. Do both parents need to be members for the child to be considered a full-fledged member of the tribe? If not, the newborn boy may some day produce new members of the tribe. If both parents must be members, then the birth doesn’t change anything—the tribe will disappear.
Second sentence said the three women were the only surviving members of the tribe.
Depends but usually not. If it was the tribe would quickly suffer from massive inbreeding and would result in death in about four generations.
Generally the baby is considered a member of what ever tribe their parents live with.