Gossip Girl - Dan and Jenny
Dan and Jenny are by no means perfect children individually. Jenny especially, as a character, tends to piss me off (then again, she's a high school girl, so maybe this is realistic). However, together, I find that Dan and Jenny are adorable siblings. They rarely fight and both seem to legitimately care about each others' well-being. Jenny, from early on in the series, did everything she could to help Dan in his lovelife with Serena. Additionally, Dan knows how to get all brotherly when someone like Nate Archibald takes interest in his little sister.
One Tree Hill - Lucas and Nathan
So these two are only half-siblings, but who would have guessed back in Season 1 of One Tree Hill that the two would become friends (and Lucas a regular presence in his now-nephew's life)? Though Lucas and Nathan started off as the biggest of enemies, it became evident that their shared common father of Dan Scott - possibly the worst father to ever appear on television - was enough to bring the two together. The two are now not just half-siblings, but genuine friends.
Prison Break - Lincoln and Michael
Let's not forget how this series started. Lincoln was arrested (but he was innocent!) and Michael, without question, figured out how to break Lincoln out of prison. This was complete with a giant tattoo of the prison-escape route. Then, Michael committed a crime himself (bank robbery) so that he could find himself in the identical prison as his brother and the escape could commence. And that was just in Season 1. The brothers since have stuck together through even more...
Supernatural - Sam and Dean
Sam and Dean Winchester have to have the best brotherly banter on television right now. The slightly more sensitive (and troubled) Sam and the more sarcastic Dean make up a perfect and wholly entertaining supernatural crime fighting duo. They've rescued each other from death more times than I can recall and almost certainly would not be nearly as successful if they weren't working as a team.
Friends (1994) - Ross and Monica
Though Ross and Monica Gellar frequently exhibited a rivalry between one another, as Monica took issue with Ross being their parents' favorite child, I still always adored their relationship. They were ultra competitive, as demonstrated in the image here from the episode "The One With the Football", but what brother and sister aren't? Though the two were not necessarily the closest when they were younger, as they aged, they became much closer, as many siblings do.
Who are your favorite TV siblings?