If you're trying to use age as an excuse for her behaving uncharacteristically (counter to the deep love and lust she's had for the MC) and stupidly, it's a pretty weak argument.
"Oh, shucks, along comes my aunt, who hasn't cared enough about me during the past 18 years to make any effort to try to contact me, and she admits that she wants to hurt my dad -- who I adore and has raised me. Guess I'll listen to whatever (lies) she has to say. She couldn't possibly have an evil agenda. Nope. She's my aunt, right? She wouldn't hurt me. But my dad, whom I've known my entire life, I can't trust. He must be keeping secrets. But my aunt? Why would she have any secrets (other than what she's been doing for the past 18 years and why she's suddenly interested in me)?"
The MC helps that scumbag Saul, and then Saul quits his job and helps Cecelia stab the MC in the back. Amanda is okay with this? Really? And Moe is okay with it too?
Amanda's behavior is, not to put too fine a point on it, absurd. Even more so than Moe (who's cozying up to a stranger and her sleazy sidekick Saul, whom Moe hated just a few chapters earlier). We're expected to just accept this crap?