
Did she sleep with one (or both) of her brothers? These dialogues had so much malice and it has been already 3 years since the end of the first arch... (⊙…⊙ )

She has slept with none of her brothers. Dion obsessively wants her because he fell in love with her crying face when she had to kill her (fake, hallucination) full blooded brother for her test. Fontaine is just a stereotypical power villain who doesn't think with his head and just wants her body. I forget his name but the younger half brother that follows her around seems to actually see her as an older sister, unlike the other two because she takes care of him unlike others in that hell house.
In history, there have been many societies that marry half siblings to keep the blood pure. As such, they saw anyone beyond full blooded siblings as marriage candidates. Incest, however, isn't guaranteed to, but tends to cause genetic deformation and mental health issues which is the main reason why it became taboo before social stigma against it started. Anyways, this is likely why everyone in this family is so fucking bonkers beyond the crazy upbringing.
Poor Collin