Brandon and Sissy - "Shame"This level represents a sibling relationship that is only inappropriate through certain filters. There's one scene in this movie in particular that makes people confused: Brandon pulls back the shower curtain on Sissy, not sure who's using his shower, and then precedes to have a conversation with her while she's naked in which he doesn't avert his eyes and she doesn't try to cover up. Through one lens, as siblings there's no reason for there to be any sexual undertones to one being naked in front of the other. Most people who have siblings might think they're more comfortable than most but that there's nothing wrong here. If you have a sibling, you have probably seen them naked or nearly naked at some point and there's no reason to think anything of it. If you don't have siblings, you might see a sexual element here that ultimately doesn't exist. If anything, being a movie about sex addition, this scene seems to serve as a contrast by showing completely desexualized nudity.
This level is about close siblings lacking boundaries that are common to lack in sibling relationships. No incest.
Simon and River Tam - "Firefly"This is the level in which eyebrows start to be raised. Simon and River lack boundaries in the same way as Brandon and Sissy while also going a step or two further. There are many times while watching scenes between them that you think, "Kiss! Oh, wait, don't!" because you forget you are watching siblings and not a potential romance. Aside from chemistry and tender hand holding, there are many scenes that set off alarms. On the lighter end of the scale, River is first introduced naked and scared and Simon holds her but you have to account for the situation. Less forgivable is the fact that River watches Simon have sex. Yes, River is kind of crazy and this is what is often blamed but for mostly everyone the idea of watching a sibling have sex is an immediate brain bleach moment. The questionable behavior is kicked up to eleven in the deleted scenes where River asks Simon if he will make a baby with her and then shoves a pillow under her shirt. She's crazy, you say, but she's also way too old to make comments like that in ignorance.
This level is defined by romantic tension and a crossing of individual boundaries regarding sex. No incest.
Brenda and Billy Chenowith - "Six Feet Under"This level is when the question of incest actually needs to be brought into play. Everything from level 4 is true here but it also gets worse. Aside from a lack of regular boundaries, as shown with Billy walking around naked in Brenda's house, and sexual boundaries, as shown by Billy photographing Brenda having sex, there's also a love confession and an attempted kiss. Billy has bipolar disorder with a bunch of side issues and a lot of the time his behavior is written off as a devotion to Brenda because she has always looked after him. However, in season three he tries to kiss her in a blatantly not-brotherly way which causes her to freak out and leave and later tells her he's in love with her. Brenda writes it off as confusion. Billy also spends her wedding in deep despair in spite of the fact that he's dating Claire. In the final season, Brenda imagines Nate telling her to just give in and get together with Billy and then has a dream where she gives him a hand job. In the case of Brenda, it's clear that this is all just a manifestation of her deepest fears regarding the way their relationship is and how Billy sees it (and as anyone who has taken Psych 101 can tell you, sex dreams about relatives don't mean anything). In the case of Billy, well, maybe he really is in love with her.
This level is mainly characterized as one-sided incest and romantic actions.
Theo and Isabelle - "The Dreamers"This level I've heard described as "everything but." Theo and Isabelle are college age siblings who sleep naked together, take baths together, play games where the consequences are masturbating or having sex in front of each other, and they have a three way with another guy. Literally, they do everything you are not supposed to do with your sibling except actually perform sexual acts with each other.
This level is solidly incest while managing to get around ever actually sexing each other up.
Cesare and Lucrezia Borgia - "The Borgias"They bang.
This level is hella incest.