Whether the stepmother's child calls the stepmother aunt or mother depends on the stepmother's attitude towards the child and the child's feelings.
If the stepmother treats her stepson as if she is a child and warms the child with her true feelings, the child will call the stepmother mother. If the stepmother does not treat the child as a child, then the child will call the stepmother aunt.
In short, the relationship between the stepmother and the child depends on the stepmother's attitude towards the child.