Why aren’t they telling him to shut up?
He made her quit her hospital job for some boring desk job, dump her best friends, and drop out of school. Now she just works that dead end desk job and has no friends. He’s over our place every single day and my parents treat him like royalty. Dude’s brainwashed them into thinking he’s gonna make it big but they’ve been together 3 years, he says he’ll marry her, but never does.
They take him everywhere for free. He eats out every day and just lounges around our house. He’s been talking junk about me to my parents, calling me a “hater” and a “red flag” because I took my sis out for her birthday and didn’t invite him. Now my parents are blowing up at me, telling me to be nice.
Why aren’t they telling him to shut up? I swear he wants me out, just like he got rid of my sister's friends. When I tried talking to her about it, she told me to back off. How do I get this loser out of our lives?

That’s one manipulative guy. You need to be strong and not let him isolate her. I would try to collect some sort of evidence of him saying or doing something that would make your family and your sister hate him.