What's the relationship between Wu San Tong (Wu brothers' father) and Lu's wife (Li Mou Cou's rival)?
Wu San Tong was the stepfather of Lu's wife, however he was fallen in love with his stepdaughter.

And why was Wu San Tong against the marriage between the two? Surely he's not a friend of Li Mou Cou?
The first question's answer this question too.