It’s a pretty important secondary plotline in all the books. The sibling relationship between Ender and his brother and sister is twined throughout.
As Amok noted, George R.R. Martin’s Dance of Ice and Fire would fill all the OP’s sibling needs, and then some. The story contains an amazing amount of family drama, and relationships between the various siblings cover the full gamut of human emotions - love, contempt, protectiveness, jealousy, resentment, superiority, insecurity, fear, and outright hatred. Not to mention incest, which is a bit of a recurring theme.
Just look at the families involved:
-
The Stark children, including bastard brother Jon.
-
The Lannister siblings, who have as twisted a relationship as any I’ve seen.
-
The Baratheon brothers.
-
Danyares Tragarean and her two brothers, living and dead (in these books the dead can be as important as the living, and not just literally).
-
Eddard Stark’s relationship with his siblings (see previous post).
-
Sandor and Gregor Clegane, who could give the Lannisters a run for their money.
-
Theon Greyjoy and his sister.
-
The three Tullys, their father and their uncle.
-
The endless Freys.
-
The Tyrells of Highgarden, who seem to be the only functional family in the series.
Have I missed any?
Katharine Kerr’s <i>Deverry</i> series includes a somewhat embarassing amount of incest, and probably fits the criteria even better than that: the whole series revolves around relationships, both kin-based, romantic and both. Starts with <i>Daggerspell</i>.
blushes has had the first book sitting on her bookshelf since summer of '02 I got sidetracked by Jacqueline Carey…and Anne Bishop…and Lynn Flewelling…and Kate Elliott…and…sighs
Granted. But I wouldn’t say that the siblings are featured, per se. They’re certainly motivation for Ender, and they have their own (admittedly somewhat absurd) storyline. You need to get past the first book and into Speaker before you starting getting the direct interation between Ender and Valentine as an intertwined main storyline.
Now it’s a challenge to me, but the only one’s I’ve come up with were the twin knights whose names I’ve completely forgotten (Blount?), and all of Robert’s bastards, who aren’t aware they’re related, anyhow. (Gendry, Edric, the girl who leads the way up to the Eyrie, etc.)
Angie Sage’s Septimus Heap books (of which there’s only one, thus far, that I haven’t finished yet) are about a family of wizards … not unlike the Weasleys, actually.
As an inside –
For what its worth, you should know that you can go to Google’s Preferences and click ‘strict filtering’. You’ll still need to modify the keywords some, perhaps adding books or novels as an additional keyword, but you’ll avoid most of what you are trying to avoid.
Google is too powerful to not advise ways of further and more precise usage. 
The Gormenghast books have some twisted sibling relationships in them.
Shadowmarch , by Tad Williams, with twins Briony and Barrick. Not quite as dysfunctional as the Majere twins, but conflict nonetheless.
I’d say Memory, Sorrow and Thorn was a better series, but it’s been a while since I read that, and I remember it as slow, but fascinating. Shadowmarch is somewhat like The Ill-Made Mute in terms of the subject matter (Faeries), but it’s quite different in its treatment.
I’m gonna throw out a little-read OOP book. Ironbrand by John Morressy. It’s the only book of his I know of that has siblings in them (three brothers). It’s the start (published order, not chronological) of a four-book fantasy series and one that, in my opinion, is much better than most fantasy series out there.
I’ll jump in and mention the Fifth Quarter, by Tanya Huff, which centers around the relationship between an older sister and younger brother. Traces of incest, but nothing too bad compared to other things I’ve heard of.
The Sandman by Neil Gaiman often focuses on Morpheus’s (the main character) relationships with his siblings.
Ok, you probably didn’t mean graphic novel, but I think this series is a must-read for anyone who likes fantasy.
A fantasy that I’m in the middle of reading is “The Fencer Trilogy” by K.J. Parker, beginning with The Colours in the Steel. I was getting a bit burned out on fantasy, but these books are very enjoyable and avoid almost all of the usual fantasy cliches. The siblings become more important as the series goes on, with Bardas’ brother being introduced partway through the first book. The family history is best described as extremely #^¢*ed up.
Word of warning though, if you like your fantasy black and white, you won’t like these books. The good guys aren’t that nice and the bad guys are much more sympathetic than you’d think at first. Parker’s got a wry sense of humor and some nice turns of phrase. If you like George R.R. Martin, give these a go.
Excellent. cackles
I love my fantasy, but I love it more without the usual stereotypes. Fantasy is possible without black and white characterizations.
… said the sado-masochistic God.
Sorry, couldn’t help it.