What should be done if the first wife threatens and curses the second wife, and only greets her in front of people, knowing that the second wife has been married for 9 years?

She should be patient as much as possible, not return evil with evil, and deal with her neighbor's provocations with silence and calmness. If she can write an impactful letter to her, that would be good, as she is not responsible for what her neighbor does after fulfilling her own duty. She should adhere to the words of Allah Almighty: (And when the ignorant address them, they say, "Peace.") and His words: (Repel [evil] with that which is better, and thereupon the one whom between you and him is enmity [will become] as though he was a devoted friend.) The best way to reciprocate her is with piety towards Allah concerning her if she disobeys Allah concerning her.