You might wanna reread cuz Suzuki said “youngest older sister” not younger sister. A few chapters before he stated that he is the youngest child in his family.
Btw does this mean that he have multiple sisters ? Potential characters to join the Sou meat hunters group?!
Now I believe the ultimate trump card to protect Sou right now is for his family to reveal that he got amnesia (although he actually isn’t) and he was a douchebag like this world normal male which will cause the girls to restrict themselves to prevent making Sou return to his “old self”