What would you do in this situation?

So my siblings have been homeschooled their entire lives. Their ages are 14, 12, 11, and 9. But my husband and I have noticed that they are not very smart, at all. Like the 14 year old is supposed to be a freshman in high school, but she is realistically as smart as a 2nd grader. Her and the rest of my siblings misspell simple words and misspell almost everything. They also just learned to read last year. When I lived with them, there was no schooling going on. My mom would start teaching, and then my siblings would put up a fight and she would stop. They often spend all day on their gaming consoles. They get candy and soda if they go to sleep before midnight, and their diet consists of pizza, chips, and ice cream. My 12 year old brother constantly threatens to kill my parents and tell them how stupid and idiotic they are. They all live in a small apartment, and so it's hard to discipline them because they're too loud and scream when things don't go their way. I just don't know what to do, because this isn't okay, right? Like does anyone have any advice for me? How do I handle this? My parents let them do whatever they want whenever they want and they walk all over my parents. There is no education or discipline going on in this home and I'm worried for their future.