Also, when the woman has a son and stepdaughters, and there's no need to mention stepdad. Her stepdaughter become the favourite over her biological son, to the point where she gives them full permission to humiliate her son.
I love this idea. Being the obvious least favourite child with a spoiled, pampered, confident and aggressive (step)sister is a great idea.
Similar to #4, this isn't directly incest related but I like the idea of a guy's daughter's boyfriend moving in and taking over the place. The kind of boyfriend that's every parent's nightmare - a dropout, unemployed, a drunk / stoner, puts his feet up everywhere and acts like he owns the place, always groping, kissing, or even fucking the daughter all in front of the father's face. Furthermore, the daughter, who used to be a studious, perfect little daddy's girl who looked up to and was very close with her dad, now starts to be mean and bitchy towards him. The dad's at his limit; he wants to throw the boyfriend out, but his wife won't let him. Rude and bullying as the boyfriend is toward the dad, he always makes sure to turn on the charm around the mom, who's very taken with him. Eventually, the boyfriend starts to seduce the wife, and officially takes over as man of the house. The dad goes from being head of the house to bottom of the totem pole. He goes to work every day while the other three stay home, lounging, fucking, and spending his money, and when he is home everyone treats him like shit.