Anthony did not have an incestuous relationship with Jackie O, and was much younger than his siblings.
Anthony also did not know of their incestuous relationship until that night and the first thing he did was run up to tell Leslie.
What was making him crazy was being in that house with his mother and having to take care of his sister. Had he remained in college he would be just fine
"Live Fast, Die Young"

I think the whole family is sort of weird in a way. He probably is sick of the incestuous relationship between Jackie-O and Marty, yet he can sort of lie about things just to have sex. Maybe he was jealous of the fact that Marty was trying to escape with Lesly and wanted to try to get her for himself.

Though this is late, Marty told Leslie that Anthony lied just to get laid. He also went to college so he's not 'extremely' sheltered, though he did drop out
It seems the insanity runs in the family. I mean I'm not surprised Anthony is weird, he has to take care of his crazy sister all the time.

This message has been deleted.