The mom flirted with the new son-in-law during the wedding party
The bride slaps the groom up and down, despite the fact he didn't ask for that.
Actually, he did deserve it. Sims have the choice to refuse flirtatious advances. They only get into trouble with their significant other if they accept the flirt.
The Romance Mod cuts down on some jealous reactions, but I don't think it would have stopped this one because the bride should have gotten jealous.
If you use TwoJeffs Casual Romance mod, it will help. Using that mod, the male Sim would have asked himself if there was anyone around who would get jealous if he accepted the flirt. The answer would be yes, the bride. Then he most likely would reject the flirt.
The male Sim still could have decided to accept the flirt even though his wife was there. This small chance was added to add some excitement and reality to the game since some people are really stupid enough to make or accept flirts even though it is risky. Really outgoing, romance Sims have the highest chance of doing something that would elicit jealousy, while family Sims usually won't take that kind of risk.