I am so hurt. May I pray for harm to befall my co-wife?

No. Feeling hurt is human and not a sin, but praying for harm to befall a Muslim who has not wronged you is forbidden.
Your co-wife committed no sin by marrying lawfully. The Messenger of Allah ﷺ said: "None of you truly believes until he loves for his brother what he loves for himself." He also taught that when someone prays for good for their brother without his knowledge, the angel says: "And for you the same." So the supplication that benefits you most is one for your own peace of heart, your husband's fairness, and the good of all the children. If you are genuinely wronged — maintenance or your turn denied — claim those rights through lawful channels, not through prayers for harm. See Relations between co-wives and Counselling.