Theoden's sisters' sons?

[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.

[Nerwen]

Originally Posted by Formendacil

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.

I was about to mention myself that I think the only natural reading of the passage in the Appendices about Frealaf Hildeson is that he inherited through his mother Hild (hence his taking her name).

As for Eomund, how "official" is it that he is a direct descendent of Brego?

[Mithalwen]

Unfinished tales I think.
Aldburg was iirc Eomund's ancestral home rather than residence ex officio of. Being Marshal of ghat Mark which of itself suggests he had high standing.independent of his wife.

[Alfirin]

I think another point to consider is that we seem to assume that Rohan runs on a Medieval European system of primogeniture, where the oldest son/male relative is the automatic heir. To me at least, Rohan is rather more similar to a Viking/Germanic type of civilization. And among them, that WASN'T necessarily how it was done. If I recall from my schooling (we actually covered this in high school while discussing Beowulf) amongst them it ran a lot looser. Basically ANY male relative had a decent claim to the throne. The King was free to pick whichever son (or other relative) he chose. He USUALLY picked his eldest son, but he was under no obligation to do so. And it was in the power of the kings nobles and council to overrule the king's decision. If they liked someone else better, he could wind up with the throne. I'm with Mithalwen on this one; had Eomer not survived, I think it entirely possible Eowyn could have wound up Lady of the Mark. (actually that could have made for a damn good story. Eowyn who is now married to the Steward of Gondor and who has renounced fighting now finds herself in a position where she is needed back home to rule (which probably means being separated from Faramir, since he is now in charge of Ithilien and presumably needs to be there, and with a title that more or less puts her as supreme military commander, amongst other things.)

[Inziladun]

Originally Posted by Alfirin

I think another point to consider is that we seem to assume that Rohan runs on a Medieval European system of primogeniture, where the oldest son/male relative is the automatic heir. To me at least, Rohan is rather more similar to a Viking/Germanic type of civilization.

The language of the Rohirrim, at least, is rendered as Anglo-Saxon, though the actual way of life they had could be similar to other cultures as well, I guess.
They, being in origin of the same mannish stock from which came the Edain and the Númenóreans, seem to have kept the same rules of inheritance.

Éomer, of course, would never have been king if not for the death of Théoden's son Théodred as part of Saruman's plot. It looks as if the plan was for Wormtongue to then arrange for Éomer's imprisonment and maybe, death, and then rule himself through Théoden.