A more thorough explanation:

UTERINE

Uterine means born of the same mother but having different fathers.

1. Sarah and John are uterine siblings because they have the same mother but different fathers.

2. The twins are not uterine siblings because they have the same mother and father.

The term uterine is used to describe siblings who share the same mother but have different fathers. This can happen when a woman has children with different partners. For example, Sarah and John have the same mother but different fathers, so they are uterine siblings. On the other hand, the twins have the same mother and father, so they are not uterine siblings.