In the book, The Outsiders, what is Sodapop's impact?

I assume you are referring to Soda's impact or role within the family. If so, Sodapop acts as a kind of mediator between his brothers. He loves both brothers..... looks out for his little brother (Ponyboy) and respects his older brother (Darry). In essence, because Soda is the middle child, his life is a bot different after his parents' deaths. He has a bit more freedom because he is older than Ponyboy, but he is also more aware of the responsibility that Darry has taken on and how important it is for everything to work out.