Don't discount Arya as a warg yet ;)
As for Sansa, unlike Arya, she understands the social structure and their place atop the food chain. The issue is, she doesn't understand it rationally, thanks to her overprotective parents. So, in a way and specially after Ned's demise, she has a point about Arya's attitude and that shouldn't be acting like she did in the Kingsroad. But she can't articulate why.
And then again, under normal circumstances neither girl should be in the Northern host. But the Blackfish needs both of them to help infiltrate into the Red Keep if the Lannisters don't accept Robb's terms.
Anyways, this is a bit of a recap/listing the position of all the pieces. War is still not quite on the walls of King's Landing
Elmar II
Elmar woke up to the chirping of birds, the crackle of campfires, the laugh of men, the yawns of giants and the putrid smell of King's Landing. Ser Tully was already awake, but most of the camp wasn't, so that didn't earn him a reprimand. Instead, he just told Elmar to go get some breakfast.
Still half asleep, Elmar Frey changed his clothes and, after a respectful salute, walked out of the command tent towards the campfires. The sky was still tinted with the red shade of dawn and all around King's Landing, Ser Brynden Tully's host was waking up to the second day of what was supposed to be a very short siege. Ser Brynden had taken up to him to instruct him in the arts of warfare far beyond the roles of a simple soldier. Night after night after the war councils, Ser Brynden would sit with him around the maps and ask him questions about the matters discussed there and provided explanations for everything Elmar didn't understand, which turned to be quite a lot.
And, as the war progressed, even the Blackfish was getting astonished by the dimensions it was taking. After the princesses left the day before, Ser Brynden received a rider with news from Rosby. The captured town had a small fishing port, as well as sailors inns and brothels, so Ser Brynden hoped to get news from other parts of Westeros, as well as the whereabouts of the Royal Fleet in the aftermath of Stannis Baratheon's death. To his dismay, too many ships were getting lost in the Narrow Sea and some of the sailors lucky, or skilled, enough to escape reported sails in the colored silks those who knew the Narrow Sea identified as the pirate ships of a Lysene sailor called Saan. No news had come from the Royal Fleet and Ser Brynden put most of the wargs to scout the Blackwater, causing the loss of a wolf and a boar descendant from Lord Twoskin's infamous gigantic boar. To his relief, only a dozen or so ships were seen, escorting fishing boats through the Blackwater.
Elmar, who had trained to become a knight all his life, dared to ask Ser Brynden about his concern for the Royal Fleet. After all, he thought, they were on land and in the same bank of the Blackwater as King's Landing. That was when Ser Brynden showed him the map, depicting all the south of Westeros with large wooden tokens showing most of the large Houses of the kingdom.
"This war is larger than any I know about. It's greater than the First Blackfyre Rebellion, it dwarves Robert's Rebellion and even Aegon's conquest. The War of the Ninepenny Kings was of a similar scale, but the Free Cities never used ravens to send message" Seeing Elmar's confused face, he continued "See here. My niece, Lady Stark, writes that Renly and Stannis died here. But before dying, Renly got Margery Tyrell pregnant. By the same time, the King broke into the Westerlands here, and smashed the Lannisters in their own ground. So far so good?" Elmar nodded and the Blackfish continued "We chased Tywin Lannister all the way to King's Landing and, after hearing of Renly's death, he sent little Petyr to negotiate in his behest. But since Margery is expecting Renly's heir and the King broke into Lannisport, the mighty Tywin Lannister, good tactician but poor strategist, has nothing to offer them"
"Ser" had interrupted Elmar "The rider said the Ironborn were the ones to take Lannisport"
"That he did, but did you forget the ravens we got from the Westerlands? Mostly likely, your family and the Greyjoys attacked roughly at the same time, but the Greyjoys sent word first"
"And that's why Ser Stevron is asking for reinforcements" completed Elmar, not wanting to appear ignorant in front of the Ser.
The Blackfish hit the map with this fingers, over the Westerlands "Exactly, and it's one of the reasons why this war is turning to damn complicated. But I'll get to that were talking about the Royal Fleet" he put a few tokens with rough boat forms over the Narrow Sea "These are Sallador Saan ships. He's a sellsail, formerly employed by Stannis Baratheon. If Tywin Lannister is the man I remember, he would have tried to recruit him as soon as Stannis died. But since the Umbers got the Castamere mines over here" he pointed at the end of the map opposite of the Narrow Sea "and Casterly Rock is under siege by Ironborns and Mallisters, Tywin Lannister can't pay him. Talking about that, never put Greyjoys and Mallisters next to each other. I don't know what your brother was thinking, but that's not going to end up well. And it's another reason why we need to get back north as soon as we can. Do you see the complexity of this all?"
Elmar would swear the Blackfish smiled when he said it, and nodded "But, what about the Royal Fleet? We are on dry land and we do not need to cross the Blackwater"
"Right, but if the sailors disembark and hit us in the flank while storm the walls of King's Landing…" he waited for Elmar's answer
"We'll still beat them. Sailors use boiled leather as armor and we will still outnumber them. Plus, we have giants" said Elmar with a smile
The Blackfish, however, shook his head smiling "The giants will be at the gates but yes, we will beat them. But, first, their presence can make Tywin Lannister present battle. And usually, you want your enemies to surrender. You don't want to waste men bringing them down" watching Elmar's incoming negative at that particular piece of wisdom, he continued "And fighting the Royal Fleet in the ground will cause us casualties, and plenty of them. So" he moved tokens on the map and added ship tokens, only to toss them aside, but removed River Lords and Northern tokens from King's Landing as well "We win, but who's coming back to garrison Harrenhall and the Westerlands?"
Still pondering about the lessons from the Blackfish, Elmar broke his fast with the men. Fortunately, the Royal Fleet was nowhere to be seen. Maybe it was as the Ser Brynden suggested, and the King's victories in the Lannister lands had caused the sailors to shy away from House Lannister. The news from the King's marriage troubled him, though. Lord Tyrell had proposed a marriage between his pregnant daughter Margery and the King, to secure the peace after the war. With the support of the Reach, the war would end but, on the other hand, one of his sisters was supposed to become the Queen. As far as they knew, it all depended on Edmure Tully: fearing ravens were lost in a storm and with the King sailing to Elmar's home to wed after that issue with the Westerling girl, Ser Brynden's nephew hurried to the Twins to break the news and make Robb Stark reconsider the marriage. Elmar had no idea what his father would do. One of the Manderlys suggested to marry him to Sansa Stark instead of Arya as a form to compensate but, the more he thought about it, Elmar didn't want it. Sansa was prettier and polite, but she was also a warg and even seeing them every day and sharing meals with some of them haven't stopped Elmar of distrusting their kind. Harrion Karstark had suggested, instead, to proclaim Tommen Baratheon as Lord of the Stormlands, reserving the Iron Throne for Margery's son, and marrying Tommen to Sansa. To Elmar, that seemed folly. Placing a Baratheon as Lord of the Stormlands meant planting the seeds of a civil war. He may be a boy, but life in the Twins always taught quite a lot of marriages and inheritances.
He was thinking about the King's marriage and its effects on his own – he thought he was digging into that 'strategic thinking' Ser Brynden praised the Young Wolf so much for by doing so – when he walked into the tent. The Blackfish was already in his best clothes, talking to the warg Maris, famous for the hawk she warged and the arguments she had with Elmar' soon to be sister in law. As expected, the Blackfish was asking for sightings of the Royal Fleet and the defenders disposition of forces. It didn't last much longer, and Ser Brynden soon dismissed the warg and ordered her and her bird to rest. And then he turned to Elmar "Have you eaten your breakfast?" Elmar nodded "Good, because we have an old acquaintance of mine to meet. He styles himself the Lord of the Westerlands and Hand of the King"