It would make sense that the Count and Baron could be part of the same family over time but I don't know how they would start that way with those positions considering we'll be taking over families on the first run.

The tribe situation seems far less likely as to be in a family it needs to be by blood or marriage. Perhaps if your Baron brother from another tribe married one of your in-game siblings then he could be brought into your family? If he had children after that they have to be surrogates from his tribe instead of his spouse if he wanted to avoid having half-breeds.