Nah, lil sis is just socially blind. It comes with being isolated in a bed with very limited interaction with people.
Her being an unwitting pawn of the mother makes a twisted kind of sense. By being sick, it ensures lil sis would never properly learn of the outside world, which in turn gives her parents full control of her understanding of the world.
It takes intent to consistently inconvenience someone the way Illya has been, and Sylvia does not have the attitude for that.