Vassal Inheritance Warning: Losing the Barrowlands to the Riverlands, suggestions? WWND?

I'm Ned Stark, playing the Crowned Stag scenario, and I'm only a few years ahead of Robert's Rebellion. Lady Barbrey Dustin has married Wallace Rushmore and had a son by him: the Barrowlands will pass to the Riverlands on her death.

As Ned, I'd assumed he'd use some diplomatic means to keep the Barrowlands faithful to the North, but how I would I go about this in game?

I have a long game in mind where the North is eventually independent, but I want to keep Ned and Robb faithful to Robert. So I'm sort of role-playing for the characters already known, but once I'm playing characters a few generations ahead, it will be more of a free-for-all.

Any suggestions? Not even specific to CKII - I find it an interesting question on it's own: What would Ned do?