Locking the sister out could get OP and the mother arrested under police policy. See https://www.pmgt.org.nz/domestic-violence-act/

Psychological abuse – for example, intimidation, threats, mind games and harassment; damaging property to hurt someone; allowing children to see or hear any domestic violence; controlling someone’s money, time, contact with friends or family as a way of having power over them.

all types of relationships; not just married couples, but those in de facto and homosexual relationships, children and other relatives of those directly involved in the abuse, flatmates or other people who share accommodation, and anyone in a close personal relationship.