My boyfriend’s sister is gross! Help!

Recently, my bf & I have decided to move back in with our parents to save money before getting married. His sister is in her twenties and has always lived at home. We get along very well, but she’s DIRTY. Like the bathroom they share is filthy, she refuses to help her mom with chores, and she even asks me to help her clean her OWN room. My bf and I regularly clean the house because his mom works 18hrs a day sometimes. I try to cook when I come over because the sister refuses to learn.

Basically I’m asking for advice on how to talk to her about this. She has friends over and I KNOW they’re judging bc I AM TOO! She needs to learn how to maintain a house like an adult 🤦🏻‍♀️

Help!