after thinking for awhile, all I can think of is, Minoriko think Shizuha's clothes is heavy and need to be taken off.And because of what Minoriko saying in panel 3, Shizuha think she gonna get raped by her own sister?
She thinks shizuha only seems clumsy because she has some kind of handicap/training like a weighted shirt or something an tries to take it off so she can see her at her best.
whilst her sister thinks she's trying to do something else that you might do when you take your cloths off hence the "sisters don't do this kind of thing"
Ack!My heart is already prepared!TugY-...you can't...!? Sisters don't do this kind of...I'm a grown woman too already....she's actually bearing some sort of personal handicap as a big sister...See me not as your little sister, but more properly as a full-fledged woman!She tripped again where there's nothing...TugFuff....Whump!Ehh!?For instance, the clothes she wears may be mega-heavy.Wait, what are you saying...!?My big sister may seem dull-headed at first glance, but that has to be because...Thnk!