Normally I hate tsunderes but there are a few girls that remain the exception. I don't know how to describe it but there is something very special about Zui and also Nana from TLR that really gets me going.
If there were even a hint of pain or sorrow in Zui's face (implying that her sister's not there anymore), I'd agree that you should feel sad.
But no, it looks more like Zuikaku's just a bit frustrated that she can't tie her hair properly.
I would just interpret it was 'full-size' portion is 'first panel', and the chibi portion is the 'second panel', aka Shoukaku-nee comes along after and helps Zui fix her hair up properly.
Aaaaand it's done, Zuikaku.Sorry for the trouble and thanks, Shoukaku-nee.