adjective per·verse \(ˌ)pər-ˈvərs, ˈpər-ˌ\
: wrong or different in a way that others feel is strange or offensive
a : obstinate in opposing what is right, reasonable, or accepted : wrongheaded
b : arising from or indicative of stubbornness or obstinacy
“It was more than perversity, of course, or the unexpected confidence she had recently acquired, that made her insist; she had indeed noticed that Gregor needed a lot of room to crawl about in, whereas the furniture, as far as anyone could see, was of no use to him at all.”
I guess it is trying to say that is is more than the stubbornness of the sister, it the new confidence that she acquired that made her insist on moving the furniture around.