[Formendacil]

There'd be more light to shed on this if we knew more about the FIRST sister-son to inherit the Mark: Fréaláf son of Hild, the sister of Helm Hammerhand. But alas, as with the children of Eómund, we know but little of that situation, and what little we know closely resembles the other situation: the king's sons predeceased their father during a great war and the crown passed to his sister-son, who also played a great role in the victory over the enemy threatening the Mark.

The chief difference from the later situation is that we don't know if Fréaláf's father was a descendent of the House of Eorl. Since Tolkien doesn't mention this, I incline to think there is no reason to think that he was, and if that were the case, it would shed light on the later situation by suggesting that Eómer and Eówyn's descent through their father was considered of little importance in the eyes of the Rohirrim, that it was their closer kinship through Théodwyn that was considered important. But, alas, Tolkien's silence is hardly definitive.