What to do if child hits you

Hello, my niece is 19 and still lives at home. My sister never really disciplined her and has allowed her to do what she wants her whole life. Now we're at the point that when my sister tells her to pick up, or do something, my niece hits her. She's punched her, put my sister in a chokehold, threatened to stab her with a knife, etc. My sister is terrified of my niece now. I'd love to share what I've learned about TSG with her, but I don't know if this case is too extreme at this point. I'd love to hear your thoughts on this.