My mom won't let my sister use test prep help from English

Hello. My mother does not allow my sister to get help with English tests. English is not our first language, but my mom wants my sister to learn the language solely from school books. 

This is a big problem for my sister. If she does not get help with English tests, she will never be able to pass them. This could lead to her being held back in school. Plus, it will make it harder for her to find a good job in the future.

My mother is not helping my sister because of pride and stubbornness. She thinks that my sister should learn English from books alone, but she should not. She needs to learn the language while speaking it as well.

When I graduated from college, we had just moved to America, and Mom had a more loyal demeanor. Now I often advise students and help with a paper, but I get a lot of abuse from my parents and am told not to interfere with my sister's studies. I am very frustrated.