David Qin and Darcy Parker in Strangers in Paradise, though I wouldn’t exactly call that a healthy relationship.

Several pairs in Elfquest: Ember and Suntop, Leetah and ShenShen, Treestump and Joyleaf, Dart and Crescent, and Newstar/Wing/Mender, though we never really see the latter trio interact as siblings.

And let’s not forget Luke and Leia, though obviously that wasn’t established early on. And I didn’t actually like that development.

Fantasy novels:
Sidonie and Alais in Jacqueline Carey’s later Kushiel books.

Miles and Mark in Lois McMaster Bujold’s Vorkosigan books (again, a late-developing arrangement) and Iselle and Teidez in The Curse of Chalion. There’s also a half-brother pair in Paladin of Souls whose names escape me at the moment.

Joram and Evaine MacRorie (and, briefly, Cathan) in Katherine Kurtz’s Camber trilogy.

Cat and Gwen in Diana Wynne Jones’ Charmed Life.

The large Weasley family in the Harry Potter books. There are other sibling pairs in among the minor characters, too.

The hugely dysfunctional Cortez siblings in Kelley Armstrong’s Women of the Otherworld series. There’s also a pair of infant twins in the later books whom we may see more of later.

Interestingly, I’m not thinking of many pairs in science fiction. David Weber’s Honor Harrington books have a few – Michelle and Calvin Henke are Honor’s cousins, and Honor eventually gets a pair of twin siblings.

Callista and Ellemir (yet another pair of twins) in Marion Zimmer Bradley’s The Forbidden Tower. Also, Donal and Dorilys in Stormqueen. There are lots and lots of others in the Darkover novels, but in those two the sibling relationships are particularly important to the plot.

Singer and Arin in Marvin Kaye and Parke Godwin’s The Masters of Solitude. That’s a relationship I’d like to see explored further if they ever got to write the third book in that trilogy (Singer is offstage throughout the second book).

Northstar and Aurora in Alpha Flight.

On the bad side of having a sibling, Thor and Loki come to mind.