According to Inusuku's notes, I think that CG belongs to one of these 2, and looking at the behavior in the game I think it is "I want to see you naked"
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Since according to the confidence, the sister first shows her breasts and still has her panties on and when you are more confident she shows herself completely naked.
This trust system and many others make this game have the "simulation" tag very highly, Inusuku really did it great.
Sadly, looking at the trailer it says that the DLC can be delayed even further until June.
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But hey, let's wait and this doesn't happen .
Now that I see this CG again, maybe I'm too much of a fangirl but wow I love Inusuku's art!!!
so many alot of new work!!!