It's funny because while I adore the Bennet sisters I didn't read P&P on my own, but I did read all the Alcott books on my own. So I'd probably have to say Little Women is my fave. It was certainly a foundational sibling story. I should say I have no siblings myself, so the ideas of big families was kind of a fascinating one to me. Having just one sibling wasn't all that interesting but several? definitely attracted my attention when I was younger. But I loved the sisters in Little Women - and crying over Beth was an annual occupation for a couple of years - though tbh I haven't reread it in awhile.
I was, as a teen, a complete addict of the Sweet Valley High twin books - esp since I'm an Elizabeth so it fit that whole self-fulfillment thing nicely. I wouldn't say it's my favorite now, but you couldn't pry me away for awhile.
(ETA: I did actually read books with characters who didn't share my name, LOL)
Hm, I'm having trouble coming up with brother-sister pairings outside of the Magic Treehouse books. *snort* there must be some, right? uh.... *draws blank* (I always liked the idea of the twins and Arwen in LotR, though that was definitely more a fanon thing in my head than much that was in canon)
OH, RIGHT. Let's address the travesty of having Trucco and Tricia Helfer play freaking siblings on Killer Women. nonononono THEY NEED TO MAKE OUT YOU DON'T UNDERSTAND