Robb at first appears more grown up than in past chapters, offering to make household decisions in his mother’s psychological absence and wearing real weapons and armor. However, he’s soon dangerously waving that real sword around in friendly company and making rash vows in response to his mother’s coincidentally correct, but essentially evidence-free conviction of Jaime as Bran’s attempted killer.
Catelyn, for her part, displays a condescending attitude toward her son, declaring him “utterly confused” and too lazy to “think things through” when really, it sounds like he’s just thinking aloud and would have come to the correct conclusions on his own. She doesn’t believe he’s ready to lead, and is (consciously or not) communicating these sentiments to him in a way that can’t be helpful, even if she’s correct. Possibly there’s some jealousy of or unhappiness at Robb’s growing independence here: no wonder her younger, and now completely helpless and domitable child is a more appealing focus for her attention.
BTW, I am willing to cut Catelyn some slack for her Bran-monomania during the first part of the chapter: while it’s not an admirable state of mind, it’s understandable, vividly and believably depicted, and an effective bit of commentary on our own society’s attitude that proper parents, particularly mothers, must be completely consumed by their involvement with their children.