Helping A Claimant

[Berghi]

So I took a castle as a vassal for the Sarranid Rebels (led by Arwa the Pearled One) and from this point I don't know what to do. I can't rule the kingdom, can I? I can't even appoint a minister. Taking in that I'm just a vassal. What am I supposed to do now? Wait for Arwa to get more lords on her side? IDK how this thing works. Also I want to marry Arwa, but idk how to do that. I'd appreciate some help. Thanks.

 

[AfLIcTeD]

You need to take all of their fiefs. If you can get lords to join you then great, otherwise they will defect anyway when they have no fiefs.
You are not the ruler of the faction, but you do get to assign who gets what fiefs. If you are playing native then you have to assign a fief if you take it, you cannot defer it. Which is why I recommend Diplomacy mod.
If you want lords to join you then you have to talk to them personally and try and persuade them to join you. Having high relations and persuasion skill will increase your chances.

Helping a claimant is quite hard to do as you will most likley be doing it all on your own. Trying to not only take their fiefs but also defend the ones you have is not easy. Unless you have a super army.