Souma was messing with Satou. He knows that Satou likes Yachiyo, and rubs it in Satou's face constantly. Souma knows that the embarrassment will double if Satou is given both of Yachiyo's chocolates. The thought of what will happen makes him blush because he is a weirdo, not because he is attracted to Satou.

As for the sisters, I preferred the opposite set. The drunk sister is great from all the teasing and poor behavior, and the youngest sister is 5'5" and 12 years old!! No wonder that Takanashi's reference frame for children's ages is 12. His sister hasn't "transitioned" over to evil like he views his other sisters. She doesn't set off his minicon fetish though because she's already gotten so tall.

I thought the other two were pretty boring. It felt like the creators weren't sure what to do with a writer and a lawyer character, so they just said, "we'll just make them as [WRITER] and [LAWYER] as possible." It felt like they had no personality in comparison to the other two. Granted, we saw a lot more of the drunk sister and the younger sister than we did the lawyer and writer, so things could change.

I did wonder why he was so afraid of getting hit in the face with a book when he's been thoroughly beaten, and particularly in the face, by Inami for weeks.