I think both Sacchan and Kotoha are different types of funny man, and Yui is the straight man. Sacchan is more wacky with her poop jokes, whereas Kotoha’s humour is more dark and sarcastic (“Are we going to kill it?” from last chapter, and this chapter’s “Was her body found turned inside out?”). Meanwhile, Yui doesn’t realise the other two are joking and takes them at face value (“Why would it be poop, Sacchan?” and “What? No, her body isn’t turned inside out!!”).
No, I think you’re translating that correctly, I think Kotoha just has a really twisted sense of humour
My translation
Sacchan says “There’s an emergency!” and Kotoha jokes with a “what, has she spontaneously turned inside out?”. Yui protests, saying that idea is scary, but Sacchan plays along saying essentially, “no, that’s totally normal, she was born inside-out”


