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If there is no court order taking away the mother's parental rights to the child, she has the legal right to take the child away from her father and stepmother.
The only way the father and stepmother can stop the child's mother from removing the child is by filing a petition in family law court to be appointed as the child's legal guardians/conservators. A court order granting the grandparents custody is the only way to stop the mother from taking the baby away.
It's common law, which is law recognized in all jurisdictions and states.
A biological parent has absolute rights to a child UNLESS/UNTIL a court takes those rights away or limits those rights.
In these cases, a court must do whatever is in the child's best interest. The judge considers the character of the parties involved, home environment/stability, financial stability, ability to properly care for the child, and any other relevant factors.
In this case, the grandparents would have a strong claim to be made legal guardians of the child given the fact that they are really all the child knows.
However, unless the grandparents seek and are awarded custody, they can't stop the mother from getting her child.