If your sister has already found some themes, then she has already done the hard part. Here are some ways to help her think about explaining themes.
Themes are what the main character learns. This is often one of the most helpful ways for middle schoolers to understand how themes work. For example, let’s say that one theme your sister has found is that grief and loss can make you stronger. Sal learns more about herself after she experiences her mother’s death. She has to accept the fact that her mother is never coming back, and move on with her life a more mature person.
Themes are the life lessons the author wants you to learn from the book. This is another helpful way to think about theme. Usually I ask the student to think about what the author was trying to teach the reader. For example, you might say the author was trying to teach the reader that everyone has to find his or her own path through life, and difficult things will happen but we will survive them.