Answer:
Louis would be considered Cat's half-brother.
Step-by-step explanation:
As a matter of fact, all three of them - Aurelia, Catalina and Louis - are half-siblings. They are not, however, step-siblings. Let's keep in mind that all of them are children of the same father, Charles. However, Charles never married any of the women mentioned. That is, he never married Cat's and Louis's mothers. Therefore, there is no "step" in this. However, even though the children's mothers are all different, they still have Charles's DNA. That makes them half-siblings. Having that in mind, we can easily say that Louis is Cat's half-brother.