After 36 years in public education that included 13 years of teaching elementary school and many years as a principal I can assure you that your question is not an easy one to answer. I believe very strongly that the real good teachers are born. I have worked with many people who were "born to teach." Unfortunately I have also worked with some who never should have entered the field.A very important step would be self-examination. One in which your sister thinks about learns about the grade levels and the characteristics of the age group to focus in on a grade. Usually there is a little inkling toward one grade or another. My wife taught 2nd grade and has no doubt that 2nd graders are the best!I feel that a good 1st grade teacher is a corner stone of a child's education and development. 1st grade teachers should be paid double.... just for teaching those kids how to read! 1st grade is vital, they all are really. K-1 you are dealing with the "babies" of the school so to speak, obviously they are the youngest and I have found those two grades very different compared to 2-3-4.Grades 2-3-4 give a teacher a little more latitude and as the kids get older and move up a grade they are much more able to discuss, reason, rationalize and communicate. These grades involve many different aspects of reinforcement and introduction of new concepts and skills. 4th graders are very active and busy people. Many teachershave told me and I agree from my time teaching 4th grade that they can be a lot of fun to teach. Their personalities are more defined and projects and seatwork are easier to oversee. It really depends on the person. Most teachers I know will usually pick the first grade they worked at as their favorite. I began in grade 5 and agree. Good luck.