Check out local libraries. Librarians can be very helpful. They can guide you to the section where family - themed or children - related stories are kept. You might find many books that have a 'like little sister' kind of relationship in the plot. Sometimes, they might also recommend some e - book resources that have such stories.
You can start by using general search engines. Enter keywords like 'like little sister positive stories' or'stories about protecting little sister'. You'll get a list of various sources, including blogs, online story platforms, and some literature websites.