why didn't Michelle just tell Cadence the truth about Stifler?

I know, not as much fun, narratively. But still. Easy fix.

Seems like if an older sister tells her younger sister "Listen, I've known this guy for years, he's a huge jerk and horndog, he has never acted even 5% this polite before, and he's only acting like this to get in your pants," the younger sister would believe and listen and decide what she wanted to do fully informed, problem solved.