Poe’s The Fall of the House of Usher.
I did a paper my Sophomore year of HS on the whole dysfuntional relationship between them being symbolized as the crack in the wall which eventually leads to the entire castle coming down. Also, Roderick and his sister Madeline are the only remaining Ushers, since they have this weird thing going on and Roderick doesn’t continue the family line, the “House” falls.
Monica and Ross from “Friends.” 
Francie and Neeley in A Tree Grows in Brooklyn?
Scout and Jem in To Kill A Mockingbird?
The Tillermans in Dicey’s Song and Homecoming?
The Pevensies?
[QUOTE=JThunder]
Quicksilver and the Scarlet Witch.
[/QUOTE]
Not in the Ultimate 'verse.
If we’re going for any siblings, I’ll throw in Buffy and Dawn, and Dean and Sam Winchester…
[QUOTE=miamouse]
Well, since comics are included, Aurora and Northstar.
[/QUOTE]
Well then, how about Death and Dream?
Sound and the Fury?
[QUOTE=5-4-Fighting]
Didn’t the brother and sister, Cathy Dollanganger and Chris Dollanganger (doppelganger, I get it) in that dreadful series Flowers in the Attic* get into this strange vibe after awhile? Being locked in the attic can do that to you, but still. Didn’t quite go all the way to incest though, just utter dependence in the absence of a sane mother.
- I read it one summer when I had absolutely nothing else to read.
[/QUOTE]
Dude, you really need to read the book again-and the sequels.
[QUOTE=Guinastasia]
Dude, you really need to read the book again-and the sequels.
[/QUOTE]
Why? Once was more than enough!
I’d mention Superman and his cousin Supergirl.
Unless it was a whoosh, it was based on his comment that Chris and Cathy’s relationship never went to incest. Dude, that entire book series was ABOUT incest.
Cathy and Chris even get MARRIED later on in the series.
Although since you mentioned FITA, I would go with Cory and Carrie, the twins. They were definetly attached to one another-but not in a sexual way.
Or what about Fanny and William Price in Mansfield Park?
[QUOTE=featherlou]
Monica and Ross from “Friends.” 
[/QUOTE]
Joey and all his sisters on “Friends.” He defended their honors like they were his wives, and it was all played for laughs.
In the movie The Uninvited, the main characters are a brother and sister, both in their 30s, who buy a house together at the beginning; they’re very much at ease with each other, and don’t seem to be considering any other arrangement than living together. However, by the end of the film they’ve both acquired love interests, with no sign of jealousy.
[QUOTE=Guinastasia]
Dude, you really need to read the book again-and the sequels.
[/QUOTE]
I guess in your haste to post (twice) you missed Post #40 and quote it included.
The OP asked for suggestions of literature, and mentioned that they are doing it for a book club, yet I keep seeing people mention TV shows and movies. Color me confused.
[QUOTE=OpalCat]
The OP asked for suggestions of literature, and mentioned that they are doing it for a book club, yet I keep seeing people mention TV shows and movies. Color me confused.
[/QUOTE]
Doper Mission Creep. The farther you get from the OP as the thread expands, the less likely the answer is going to actually fit all the criteria of the OP. If the thread spills over to a second page, the likelihood of perfect criteria fit drops to single digits.
Edited: Corollary to Doper Mission Creep - The above definition should not be assumed to preclude posts in the upper portion of the first page of the thread from being far afield. This is simply Doper Eagerness leading posters to reply without reading the whole OP.
I used a broad definition of fiction. 
[QUOTE=Reno Nevada]
The Wimseys have a good sibling relationship depicted in Clouds of Witness.
Do you want specifically brother and sister, or any sibling?
[/quote]
Well, for book club purposes, we’re specifically talking about brother-sister relationships. But the thread may go wherever zeitgeist takes it.
[QUOTE=5-4-Fighting]
I guess in your haste to post (twice) you missed Post #40 and quote it included.
[/QUOTE]
:smack:
Sorry 'bout that.
But I think the Dollanganger twins definitely fit the definition the OP stated. As well as Chris’s and Cathy’s devotion to said twins.
Well, the Dollanganger saga was a family tradition: There mother Corrine married the man she thought was her half-uncle, son of her father’s brother and his young wife. Of course, it was later revealed that they were half-siblings, Christopher having been fathered by Malcolm, Corrine’s father.
[QUOTE=Guinastasia]
:smack:
Sorry 'bout that.
But I think the Dollanganger twins definitely fit the definition the OP stated. As well as Chris’s and Cathy’s devotion to said twins.
[/QUOTE]
Yes, I agree there. The mistake I made is thinking I remembered the details of some trash I read over 30 years ago and vomited back up at the time.
[QUOTE=Annie-Xmas]
Well, the Dollanganger saga was a family tradition: There mother Corrine married the man she thought was her half-uncle, son of her father’s brother and his young wife. Of course, it was later revealed that they were half-siblings, Christopher having been fathered by Malcolm, Corrine’s father.
[/QUOTE]
Um..what, exactly, is a half-uncle? 
Unless you’re using it to mean “half-brother of one’s parent,” that phrase seems pointless to me.