It only took a moment.

Catelyn Stark had argued with her son when she'd found that he'd brought his bastard sister south with his army. She'd rid herself of half the pair when the boy had gone north to the Wall, and she'd hoped that she'd at least been able to avoid the other half until the girl could be married off to a house minor enough that the prospect of Stark blood in their house would count for more than the fact that it came by way of a bastard.

But her husband's bastard twins had grown too close to her own trueborn children. They'd never been learned their proper place in the world. Instead, they'd been taught to give lip service to their station - calling her husband Lord Stark in public as though that obscured the gifts and the lessons and the occasional slips of Father in private.

Catelyn couldn't get away from them. She had no idea why Robb had brought the girl with him, or why she was always at his council meetings, or why she spoke so freely and challenged even the greatest lords of the North when she thought they were wrong. She had no idea why many of the lords - not all, nowhere near all, but many - even seemed to listen to her, or why Robb glared at those that scoffed.

It was building resentments. It had to be. Catelyn had never thought that she would miss Jon Snow, but even he would have preferable to his sister. Great lords didn't like bastards pretending to high status in their midst, but they particularly didn't like bastard girls who claimed to know about combat and troop movements and strategy. She knew this, even if Robb did not.

It happened on a day when she was particularly smarting from her son's insistence on treating his bastard siblings as though they were trueborn, following a conversation with him that had become downright nasty. She'd been quietly making inquiries about whether there would be anyone willing to marry the bastard and take her off their hands when Robb had found out.

He had not appreciated it.

"Two of my bannermen have come to me with an interesting story," he'd said, his voice soft. Grey Wind was pacing behind them; he was restless, which meant that Robb, despite an outward appearance of calm, was restless. "They said that they'd heard that I was making inquiries into marriage for my sister."

Catelyn pursed her lips together but didn't speak. If his bannermen had indeed come to him, the platitudes she'd gotten thus far may not have been as empty as she'd thought. That was promising.

"I haven't made any such inquiries," her son added. The word inquiries slid off his tongue as though there was something deeply distasteful about it. "Nor have I asked anyone to do so on my behalf. Do you know anything about this?"

Now she sighed. "If it could help solidify your alliances -"

"No." His voice was flat. "I need her here."

"Robb -"

"I need her here," he repeated. "I've told them that it's not something I'm considering right now."

"You had offers?"

"It doesn't matter. I need her here."

Catelyn knew her son well enough to know that that was the end of the conversation.

She was still smarting from the abrupt dismissal several hours later when she heard the clash of steel and a girl's laughter. The former sound was far from uncommon in a camp of thousands of soldiers; the latter, however, was more unusual. When she peered around the tent, she saw her son sheathing his sword and offering his hand toward the girl, who was sprawled on the ground, a sword still in her left hand.

There were more men watching than Catelyn was entirely comfortable with. This wasn't a private affair, and no good could come of a woman publicly learning to use a sword. Robb would have done well to remember that, but he'd always been blind where her husband's bastards had been involved.

She heard a soft chuckle behind her. She glanced back at Rodrik, whose eyes were fixed on the scene. After a moment, he met her eyes and smiled apologetically. "I'm sorry, my lady," he said. "For a moment, I…" He trailed off.

Catelyn glanced back at the circle. "You're improving," she heard her son say. She looked back at Rodrik before she could hear the girl's response.

He didn't need to finish his thought. They both knew who was thinking about. "She has her blood," Rodrik said apologetically. "That's all."

Catelyn barely acknowledged him as he strode away. Something about the phrase he'd used was nagging at her.

Her blood. That was it. Her blood.

Her memory took her back to Winterfell, back to her husband. He'd used that phrase, too. My blood, he'd said. She'd thought nothing of it at the time, at least not beyond the dull ache in her gut that she always got when she thought about Ned and the woman he'd sacrificed his honor and vows for.

Now something about it stuck with her, burrowing its way into her skull. My blood, a little voice said. My blood.

She couldn't have said why the strangeness struck her at that particularly moment, but it did.

My blood.

She couldn't remember a time her husband had ever explicitly told her that they were his son and his daughter. She could remember him alluding to it, saying it to others, and above all never correcting anyone who called them his bastards, but he'd always skirted around the words when she'd asked him. She'd thought that it was simply shame.

Maybe it had been something else - or, at least, a different kind of shame.

When her mind finally put the pieces together, she felt no less stunned than she had when she'd taken a stumble down the stairs leading to Winterfell's crypt, early in her marriage to Ned. And yet the more she thought about it, the more the pieces seemed to fit. They weren't his bastards at all.

They were his sister's.

A/N: When Jon tried to desert, I started to think about the dynamic he would have introduced to Robb's campaign if he'd been there. At the same time, though, I really do like Jon at the wall, and the idea of a girl makes the dynamic even more fraught when it comes to Cat, so here it is. :) I may revisit this character (whose name is Aislin, though I specifically chose not to let Cat use her name) at some point, but I'm currently undecided. Thanks for reading!