Orange hair is not bio related to the father's and their brother might not be either. But since brother and him kinda got similar features to the father (like the yellow eyes), I'm assuming their bio father might be the uncle or somehow also related to the duchy, but not in a way that would give them inheritance, expecially for orange hair (and expecially if Duke found out). Little does orange hair know that MC is actually bio related to the duke, and I'm assuming that the orange hair mother has a bigger hand in MC's death than MC's dad. She might have been extra keen on getting rid of MC since it sounds like her sons (and potentially daughter that passed away) are not related to the Duke. That's all I got for now, and just in case, it is all my theory.
Nice theory ε=ε=(ノ≧∇≦)ノ
So who has a theory???? Cuz I don't aaaand I need it
Σ(っ°Д °;)っ