To clear up a couple of things about your half-brother: the aristo MC doesn’t have one, and the helot MC will shortly have confirmation from Joana that theirs died in infancy.
Sorry to waste an opportunity for family drama, but my plan is for this to remain an aspect of your dad’s story, not yours (directly, anyway).
If you’re an aristo, your father mentions his mother’s siblings while chatting with Horion, and that line of (former) Lastname-atou or de Surname will pop up later in the story. Other than that, I don’t currently plan for the game to include family members other than with your father. Family relationships among the helotry are atomized by sales and early deaths; you have a host of “cousins” in the camp but no one with a close known relationship.