@Purplelibraryguy
Okay, a momentary reaction . . . but her legs are staying jellied? How does that even work?
I'm putting "it's a joke, man" aside to look deeper at this. XD
One answer might be acute stress reaction. They are usually triggered by instances where your life or your close kin is threatened (or even lost, in the case of kin), but then the weakness can last a long time. (So maybe she technically could walk, but not very fast and she preferred not to.)
Another answer is that it was mostly short-term, but triggered other problems of nausea or low blood sugars that just made her recover so much slower that she's still a bit woozy and prefers not to walk.
A third answer is "it didn't last very long and she's able to walk again, but she's giving herself some CBT to calm down and like her brother again, while also letting him feel deeper that even if she was childish, it still wasn't that much of a good idea to stoop to her level"