My sisters are 16. They are my full sisters. And I love the…

My sisters are 16. They are my full sisters. And I love them very much. But my dad has a new girlfriend with 3 kids. 17, 11 (autistic), and 10. Aside from the 17 year old these kids are horrible. They have no manners and cuss more than I ever have and I'm 21. They don't listen. It's like she's never disciplined them they're whole life... Why couldn't my dad find a girl whose kids were already grown.