[@Masterkeun] I can see you're probably not finished with the sheet, since there seems to be a lot of room for further detail but there's a few minor points I want to bring up. 

1) Since this draws heavily from established canon, you have to take into account House Frey's genealogy. There were a looooooot of Frey heirs. Sons, and grandsons, and greatgrandsons, and bastards, and grandbastards and so on. Edwyn and Black Walder were the main contenders and there was probably a a lot of others, and a lot of Freys probably died, but it's next to impossible that there wouldn't have been a male heir to House Frey. So you'd kinda have to come up with a specific lineage with the Freys you want to play. Also in canon, I do not believe there are any swamps outside of the Neck mentioned. The only swamp nearby the Riverlands would be where the crannogmen live, and they incidentally have a feud with the Freys. If they Freys had to live there... well let's just say I don't see the Freys living in a swamp. Alternatively, they could still hold the Twins but they could have had a lot of hostages taken and those complicit in the Red Wedding executed. Basically what happened to the Greyjoys when they rebelled. 

2) It's only in very specific circumstances that a woman would lead a House. Dorne is the exception to the rule, but all other notable female rulers we have in the series are either widows of the Lord or the last of their name. Or they live in environs where people are a little more accepting of female rulers, like the women of Bear Island or Asha Greyjoy with her sailors. It is extremely unlikely and implausible that Mable wouldn't have any eligible male relatives who would take control. In most of Westeros, females only inherit when all other legitimate male heirs are not an option. With the information I have, Mable's twin brother would be the rightful ruler of House Frey. But again that's assuming all of the canon Freys just all died, which is unlikely. I also find it unlikely that the Freys especially would tolerate a woman as leader. I think your original concept of having Mable apart from the leadership, as a sellsword might work better. 

3) The Mountain Clans are seen as barbarians by the whole realm. Tyrion getting a deal with them was pretty much a once-in-a-lifetime event. Walder Frey certainly could not and probably would not have made the same kind of arrangement. If you need a leader of the Frey military, it'd be better to use an actual Frey since they're so many of them and the House Sheets are for family member PCs. Servants and vassals would be NPCs.

4) There needs to be a lot more detail. What are the trueborn sons up to? Why would Jon refuse to help the Freys loyal to the crown? Why would Mable still be loyal to the crown if Jon didn't help her? All in all, there needs to be quite a bit more explicit details and exploration of motivation and personality. 

I understand that it's probably a work in progress, that's just my thoughts on what I see so far. This is not meant to discourage you at all. 

[@Nightwing95] Great sheet, you are accepted. My one minor nitpick is that I don't think non-rulers get the roman numerals after their names. IIRC that only happens if someone becomes ruler and their namesake was also a ruler.