It’s her job to teach her students. It’s not her job to teach her nephew and appease her entitled sister-in-law.

Tons of parents think that any teacher can teach any grade or subject. While some teachers have transitioned from teaching middle school to high school to elementary school across various subjects, many remain in the same couple of grades, teaching one or two subjects for most of their teaching careers. Unless you’ve actually been a teacher in a classroom setting, you don’t know how vastly different a 1st-grade classroom and a 3rd-grade classroom really are, even though the kids are only two years apart from each other. Also, math and reading require incredibly different pedagogies. If a teacher is only trained in teaching older elementary schoolers’ math, they aren’t going to be successful in teaching a five -or six-year-old how to read. 

A jack of all trades is a master of none, and most kids benefit from having different teachers who are experts in their main subject, instead of master-of-none teachers who aren’t particularly skilled in any one subject, or a math teacher who is teaching reading because there are too many math teachers and not enough reading teachers.