Which is your favorite fictional sibling duo?
This one was actually hard because I had to pinpoint which fandoms had siblings, while narrowing it down to sibling duos.
01. Buffy and Dawn Summers (Buffy the Vampire Slayer).
When I think about sister duos, I automatically think of them. Their relationship that formed throughout the series was so great. Dawn's appearance on the show during the fifth season was the centric plot point of the season, however that didn't stop Buffy from loving her and doing whatever necessary to protect her little sister. One of my favorite scenes between them was, after Dawn realized that she was the Key and was only recently created, believing that she wasn't a real person and nothing matter, Buffy proved her wrong. "It doesn't matter where you came from or how you got here. You are my sister." It's such a touching and defining moment about their sisterly bond. They may annoy each other, but they love each other, no matter how their relationship came to be.
Also, Dawn herself grows up to become a wonderful and strong young woman, and that wouldn't have been achieved hadn't Buffy been such a loving and understanding older sister, who looked after her when their mother was gone. Buffy was someone that Dawn looked up to, and Dawn was someone that Buffy was proud of for becoming her own strong, independent young woman.
02. Justin and Alex Russo (Wizards of Waverly Place).
While there are three siblings in Wizards of Waverly Place, Justin and Alex's relationship are the primary focus on the show. They couldn't be any more opposite of each other. Alex is the troublemaker and free spirit of the family, Justin is the responsible one; she's reckless and carefree with magic, he is a stickler for rules and following directions, etc. However, despite this, they perfectly complement each other. When Alex makes mistakes or is in a tough jam, Justin is there to help her fix it, and vice versa. Sure, they will bicker and try to one-up each other on many occasions, but when shit gets real they won't hesitate to work together to solve the problem. They're pretty much inseparable when it comes to each other in that regard. Justin and Alex are the ultimate brother-sister duo, in my opinion. Because they drive each other crazy, and yet they love each other, and it shows through their actions. Also, the amount of growth and maturity through relationship goes through as the series goes on is absolutely beautiful.
(And yes, as I mentioned previous I unabashedly ship them, but their chemistry is just that good that even the showrunners have nudged a little bit of that throughout the series, and definitely in the movie.)
03. River and Simon Tam (Firefly).
Simon risked everything, including his status on his home planet and all he achieved to become a successful doctor, to save his sister and be on the run. He is very protective of her. River, in turn, is also protective of her brother and has a lot of faith in him, so much that in the movie she takes matters into her own hands to save his life in exchange for all the times he did for her. Everything about their relationship is absolutely beautiful and lovely and you just can't not see how much they love and care about each other. Simon is River's entire world, just as much as she is to him. I cannot tell you much how I adore their relationship.
(And yes, I also ship them as well. No shame.)
Honorable mentions: Casey and Derek from Life With Derek and Lucrezia and Cesare from The Borgias. Ironically, I also ship them as well. Oop, lol.