yup....I don't get the joke.

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?

Ironically, it's Minoriko who would have the excuse for being clumbsy, having two left feet and all.

BrotherJohn said:

yup....I don't get the joke.

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"

Not sure what else to see to be honest

I thought Shizuha's clothes were made of leaves.