I can't deal with my sister anymore

She (34F) was absolutely awful to me and my parents this past few days. Just absolutely blew up yesterday because we told her to wait a few minutes before eating. She screamed bloody murder and brought up some past family drama and said some really hurtful things. She has a history of defiance and making scenes as a full grown adult.

My parents are in their 70s and we cannot take this anymore. I know she needs to go to therapy. But it’s not working as she has tried it in the past. What on earth do we do. Is it OK to cut her off?

TLDR: Sister is a grown adult and still acts very immature. How do I handle it.