I'm writing and I need help with a term. So Aurelia's father (Charles) wasn't married to a woman and she had a child (Catalina).

Later on, Charles had a child with another woman who he was not married to, and she had Louis. Would Louis be considered Cat's step-brother? If not, what's the correct term?

Louis would be considered Cat's half-brother.

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.

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