What do you think of this plot idea for a book?

I never condone incest (even through marriage) and pedophilia (even though he is reaching age of majority) in books I select. That is also true of many publishers. If she is being seductive toward Eli without ever being physical is a different thing. Her being flirty with his friends too may show her in a less positive light while Eli can then maintain the sympathy of the reader. She may go ask him to take a look at her car and lean down so her cleavage entices him or stand above him while he is under the car. She may give him fleeting touches when he carries groceries or boxes for her but nothing more. She can call him endearments even brush close to his body and remark about his reaction. But I'd keep it as non-interactive as possible to reduce the amount of backlash you can get.