Also, I have the culturally-ingrained vomit reaction to the concept of incest. I am aware that sometimes stories feature incest in a believable fashion, but in those cases I want some acknowledgement that it's unhealthy and a reaction to emotional damage. I resent the implication that all 'important' relationships must in some way be sexual. I like knowing there are people in my life with whom I have an easy familiarity and anything remotely relating to sex is simply not a factor, and I like seeing that reflected in literature.
So just off the top of my head, a discussion of several sibling pairs in fiction. Will include spoilers for Huff's Quarters quartet, C.J. Cherryh's Cyteen, Seanan McGuire/Mira Grant's Newsflesh trilogy, and Sarah Rees Brennan's The Demon's Lexicon trilogy.
-Tanya Huff, Sing the Four Quarters, Fifth Quarter, and No Quarter: I love how the set-up of the first novel leads you to think the main relationship to be explored is between Annice and Pjerin, with the king as a two-dimensional rule-bound villain. Then you discover the king is a real character who was simply very hurt by his little sister and over-reacted in a way he couldn't easily take back, and the emotional arc of the story is a brother and sister repairing their relationship. (I'm sorry I can't remember his name, it's been about fifteen years since I read these.)
Fifth Quarter and No Quarter are a duology, and a bit more chameleon-like than the first book. It starts off with the relationship between siblings Vree and Bannon, goes into a love story between Vree and the third major character, then swings back to the sibling bond. I like that Vree's romantic relationship is important to the story and her development, but ultimately that arc isn't her core conflict - resolving her screwed-up relationship with Bannon is. I'm okay with the incestuous twinge at the start because Vree is aware those feelings aren't healthy, and although it takes extraordinary and unpleasant events for her to move beyond them, she does and by the end of the second book she has a partner and a normalized (such as it can be) bond with her brother.
(The Quartered Sea, as I recall, is not about siblings and therefore not relevant to this discussion.)
-C.J. Cherryh, Cyteen: I finally got around to reading this last year, and I understand why it's a classic. But my enjoyment of it was marred because at the end all I could think was, "Well, those things were great, but WTF incest??" I get how and why it happened - Justin was abused for years, starting with sexual abuse as a teenager, and for very understandable reasons couldn't stand to have anyone but Grant even touch him. I love the absolute trust Justin has in Grant, I love how protective Grant is of Justin, but I'm not okay with that shift in their dynamic. It's especially jarring because Justin fights for years to have Grant recognized as his brother, while everyone around them insists azi can't be real family, and then he just all of a sudden gives up that fight and starts sleeping with Grant instead. The narrative is very clear this developed due to abuse, but unlike Huff's Vree, Justin's healing does not include moving beyond it. I strenuously disagree that turning one's brother into one's boyfriend is a good long-term coping mechanism. I described my problems in general terms to Clare, who hasn't read this, and she suggested that some people already don't consider same-sex relationships as 'real' or 'healthy,' and therefore it might be easier to do this with them than with a straight couple. I don't know if that's a factor, but it's truly unfortunate that this prevents me from loudly recommending this book without reservation. I do recommend it - but seriously, WTF?
-Seanan McGuire (under penname Mira Grant), Newsflesh: I have such a high opinion of McGuire because of her October Daye books, so I was quite disappointed that she also turns up on the yay-incest list via this trilogy. I know most people love the dynamic between Georgia and Shawn as the best thing about the work, and I partially agree. I liked how the presentation of the dynamic started out - the first chapter of Feed felt very believable to me as a close sibling relationship between two well-drawn characters. I was less happy that Georgia acknowledges early on that she and Shawn being joined at the hip is not something that would generally be perceived as healthy - but she knows all those people are wrong (or at the least, does not care). Even were the relationship platonic, never being separated from your sibling for more than about a half hour at a time is not good. The incestuous nature of their relationship isn't explicit until midway through the second book, but there were hints beforehand so by the time of the 'reveal' I had already resigned myself. There's a scene in the second or third book where Shawn and Georgia's mother hints she might have been aware what was going on, which is particularly screwy. I know she's portrayed as exploitative rather than nurturing but come on, there's a line there. Like Justin and Grant, I understand to an extent how this came about, both the incest and the co-dependency - the same dynamic of 'we've always had each other, we've only had each other,' but again, I want a character arc that includes learning to develop different ways to cope (see: Vree and Bannon). I do not accept the excuse that it's more okay because they're not biologically related - plenty of people in my family have no genetic connection to me but they are absolutely my family. So I did like the love and trust between Georgia and Shawn, the quipping at each other, and the way they complement each other, but I just can't be as invested in them as characters and their relationship.
-Sarah Rees Brennan, The Demon's Lexicon trilogy: First and foremost, these books are about siblings. Ultimately the entire trilogy is about Nick and Alan and the relationship between them, but it also highlights Mae and Jamie, and Sin and her young sister and brother. And zero incest! I will admit all three books made me cry at least once because of scenes with Nick and Alan. And I love that unlike McGuire, Rees Brennan doesn't hesitate to delve into how their love is so beautiful and so fucked-up at the same time. Echoes of Greek - greatest strength, greatest weakness. I love how the books show that a love so intense and all-encompassing is also by definition so selfish. Characters in other works face conflicts in which they have to choose whether to sacrifice the whole world to save someone they love, and in this text that choice was made long before the books start, clear-eyed and without hesitation, and we see it play out again and again. They hurt each other as well, also again and again, and the reader can see both sides of the miscommunications, even if you occasionally want to shake one or both of them. I like how easy it would have been to put the blame for every miscommunication on Nick, but instead Alan is culpable as well.
I like seeing at the end how much each of them has grown and how much they haven't. Both of them developing real relationships, friendship and romantic, with other people shows growth for both of them and allows for more. Co-dependency still rules the day, though, and the ending is ambiguous whether they'll continue bettering their understanding of each other or fall back into old patterns. (As a note, I was very invested in the romantic aspects in the books, and I appreciated that they were important while not being the primary relationship type at issue.)
The first SRB book I read was Unspoken, and at the end if you'd asked me what it was about I'd've said, "This is a book about two people who love each other so fiercely and communicate so badly, and support and hurt each other in equal measure." I find it funny that that largely sums up Demon's Lexicon as well. Very different works, playing with different tropes and genres and what-if spec-fic questions, but both very concerned with the power of words and storytelling as well as their failures, and with love and trust and self-definition. There are worse themes for an author to be attached to.