Reddit mentions of Theories of Childhood: An Introduction to Dewey, Montessori, Erikson, Piaget, and Vygotsky

Your sister should get certified as a teacher, no doubt. This will let her become a teacher. But we don't need more teachers, we need more great teachers.

Some things that more teachers need to do (not just Deaf school teachers):

• Get a strong pedagogical foundation. That is, the methods and psychology of teaching. The school system in America has been static, because not enough teachers know how to teach. There has been ample study on the psychology of how students learn, and how best to reach them. Most "alternative teacher certification" programs have no focus on pedagogy, just basic knowledge. Knowing facts does not make you a good teacher. You have to learn how best to develop knowledge in students, it does not come naturally. If your sister does not have access to a good pedagogical program, have her seek one out. Look up constructivist learning techniques, inquiry-based models, and findings from Piaget, Vygotsky, and Erikson.
  
• Have an advanced certification-level of experience with the language. When it comes to schools for the Deaf, some schools will required their teachers/interpreters be certified at the state or national level. Others will just require that they have "achieved the proficiency level required". If she's a strong signer, certification isn't as important, but getting certified as an interpreter will open more doors. If she were teaching by speaking, say, English, she should have an expert ability to speak English, not just a cursory knowledge of it. The same should hold for ASL, or technical "languages" demanded by the subject.
  
• Find a school where she agrees with the policies. If I had a Deaf child, I would never send them to a school where signing was forbidden. The classroom should be an environment where students and teacher communicate openly and freely, and placing barriers on communication makes no sense educationally. If the point of a school is to teach them to interact in a hearing world that fears and does not accommodate them, and she's okay with that, then she can find a school that suits her principles. If the point of a school is to teach students and allow communication, we should not forbid the use of any tools or languages. When it comes time to interview at schools, she should ask them on their policies. If they are policies she doesn't agree in, she should make it known at that time. If they do not (or cannot) acquiesce, it is not the school for her.
  
• Keep that passion for teaching. There are few excellent teachers in this country. Teaching is a demanding profession, and is often downtrodden. If you are not passionate about your subject and students, never get in to teaching to begin with. If you keep that passion, it will be one of the most rewarding professions possible.
  

Step wise, I'm sure you know about Stepmonster (on the side bar).

When it come to teaching kids/child development, I love Vygotsky. It's everywhere in all of my posts. He's the one that focuses on giving your kids the gift of confidence to help them learn and grow. If you're not yet set on a child development theorist, you can pick up a book like this for an introduction. If you have more of an idea of what you're looking for I'm happy to search deeper for you to help you find the right fit.