Yuuko is a very fun little sister. A bit annoying, maybe, but I'd honestly never expect perfect obedience from little siblings anyway, as much as it may be a misconception. I speak from experience being the youngest brother in my family.
Being unreasonably annoying to my big brother, hell yeah! Good ol' times.
Plus her being there in this episode really triggered some very nice interaction between Nanami and Sorata. The hold-on-stairs scene was probably the runner of the episode for me. As a NanamixSorata shipper, this was most favourable.
I'm gonna open my eyes especially wide next episode... gonna see dat frames per second on the bouncing boobs.
Just kiddin'... maybe.