Song Wu killed Pan Jinlian when he knew Sun Erniang. Why is Sun jealous?
I think it should be Yulan, the maid of Zhang Dujian's family. Zhang Dujian betrothed Yulan to Song Wu. And Sun Erniang especially likes Song Wu. So if you choose a rival in love, Yulan is more reliable.
The old man under the moon represents one, a small red line, or two, because he is a matchmaker and wants to combine two people.