It's not about being told what to think, it's about trust. Hopefully there are people in your life that you trust, whether family, or friends or a spouse, so that you know what that's like, but the point is that these people have been around for a long time, and would point you in the right direction and wouldn't intentionally lead you down the wrong path. Now imagine a third person enters the picture that you don't know, but this person you trust does, and they tell you, 'that person is bad news'. Now, what free thinking individual concludes 'golly, this new person must be more reliable than this other person I've known my entire life and has never done me wrong before'.
I get that some people don't have good relationships with their parents or find it hard to trust anyone at all, but that's not Amanda. Prior to her aunt showing up she's not shown to have been badly treated at all, and has no reason to think that her dad was manipulating her for any reason. Even sexually, Amanda is the one making all the moves. But, it's not like I don't expect Amanda to even talk to her aunt. What I find so mind-numbingly bewildering is that, when confronted with this new information from this untrustworthy source, she doesn't confront or even discuss it with her dad. She doesn't even tell him she's been to see her Aunt. Instead, she gives him the cold shoulder, stops showing up to work, moves out, and then, when he finally shows up with a gun and ends up in a cell, shows up and tells the dad she's happy she can talk to him now. It's absurd.