Before I start, we know it was wrong.
Sister is in a coercive and controlling relationship. Her partner is financially and emotionally abusive. Belittling, controlling, takes all her money and berates her for not earning more, opens her mail etc. Their 5 YO is very sick with cancer, having chemo and he moans about having to sit in the hospital with her, or look after the kids whilst sister works. You get the gist, he's not very nice.
Older sister had enough today, after learning of more abuse from him and he'd kicked their dog. Older sister threatened him with violence.
Has anyone reacted emotionally and had to deal with the police? None have been called, but there's still time and wouldn't put it past him. She shouldn't have done it obviously, but admits it was very wrong.