On the importance of convincing motivation for a character’s choices
To an author whose well imagined character does implausible things to serve story

The first thing I want to say, because I’m going to come down hard on you in one area, is that you are a superb writer with a strong plot and a really interesting heroine.  

So what’s the problem?

Motive. Why on earth does Alice, as you’ve characterized her, hang out with Preston? Get into his car? Let him go on doing the repulsive sexual things he does until the inevitable rape? After a while we learn that she loves being treated like a grownup, but by Preston? Loves that enough to put up with what goes with it? It’s just too hard to believe.

There’s no evidence in these pages that she likes him, enjoys him, thinks there’s anything good about him. As for his romantic and sexual moves, not only do they disgust her, but he’s her sister’s husband!  It’s hard enough believing her sister would marry this older jerk with the ghastly manners, bizarre behavior, and receding hairline, but we can assume she’s dumb. Alice we know, and her letting Preston get by with murder with her before he literally murders her sister just doesn’t make a lick of sense. 

There are two things I can think of that will keep this from making the story unbelievable. First, you have to give Preston some appeal. Make him fun, make him endearing in some way, give him a good side. Right now he’s a cartoon villain, and no child of any age with a lick of sense would go anywhere with him after the first time. And if he forced her to, she’d tell somebody. Or if she didn’t, she’d act so traumatized that somebody would notice and question her until she spilled or they got suspicious. But this issue is entirely solved if he’s got some good points to balance the awful ones.

Second, you need to have Alice get something out of their encounters that’s important to her. It’s not enough just to let us know that he treats her like a grown-up–show him doing that, show her reveling in it, have him be sweet about it. Let us know from the get-go how bad things are at home, which will set up her need for the “love” he showers on her. 

Of course, you show us her mounting awareness that what he’s doing–what they’re doing–is wrong. Really wrong. The older she gets the more she realizes how wrong it is, the more she tries to extricate herself from the relationship the more threatening he becomes, and so on. 

If she’s going to survive this ghastly childhood, you might consider having Preston leave her alone for periods. Or having him be away for some reason for a while. Or simply having him not be “at” her all that often. 

Alice is a wonderfully developed character who’ll be totally convincing–if you can just make the reader understand why she lets Jack do what she does.