Do I need to be on good terms with my co-wife?

You are not obliged to become close friends, but you are obliged to keep the adab owed between Muslims — not wronging her, backbiting or cutting ties of kinship. A co-wife is a sister in faith; the Prophet ﷺ forbade us to envy, hate and turn our backs on one another. A good relationship makes matters concerning the children, the rotation and family occasions much easier, but it can be built slowly. Start with small things: giving salam, not questioning behind her back, and respecting each other's boundaries. If that is too hard, a polite and distant relationship is enough. The husband must not set his wives against each other.