"Wincest" is totally canon in Supernatural. Not in the hot-and-heavy way that Sam/Dean shippers, unperturbed by the taboo, wish, but in the way that the cultural, fan fiction-driven practice of "Wincest" exists, bizarrely, within the show's universe. For the uninitiated, that's "Winchester," the protagonists' surname, combined with "incest."
Supernatural is far from unique in attracting a particularly high amount of "slash" (same-sex) romantic/erotic media from its fans. (The phenomenon actually dates back to Kirk and Spock in the early days of Star Trek, when fans would swap homemade zines at conventions featuring the fictional pairing.) But, Supernatural has become particularly infamous for it, thanks in part to the series wholeheartedly embracing this niche pocket of fandom.
As early as Season 1, Supernatural was cognizant of the homoerotic undertones of its two attractive, single leading men, who were frequently asked how many beds they would require when checking into a motel. (The best thinly coded reference is perhaps Season 2's "Playthings," in which hotel owner Susan asks the clueless brothers, "Let me guess, you guys are here antiquing?" Or, more explicitly, the sadistic angel, Zachariah, describing the brothers as "erotically co-dependent" in Season 5.)
The running gag just about skirted the inherent homophobia that comes with most "offended at being assumed gay" jokes by having Sam use it to question Dean's fragile masculinity: "You are kind of butch. They probably think you're overcompensating."
These are the kinds of winks and nods post-modern audiences have become accustomed to, almost to the point of expectation, which is probably why Supernatural felt it could up the ante, brushing further and further against the fourth wall until it outright smashed through it in later seasons. Season 4 was the turning point. In "The Monster at the End of This Book," Sam and Dean come across "Supernatural" pulpy novels that eerily chronicle their own lives, and then track down the author.
In the process, they stumble across the books' largely female fanbase online, with an accompanying library of erotic Sam/Dean fan fiction. "They do know we're brothers, right?" a stunned Dean asks. "Doesn't seem to matter," Sam answers disdainfully. It's notable that the first episode to officially induct the concept of "Wincest" into canon was written by two women, Julie Siege and Nancy Weiner. The idea was then doubled-down on in "Sympathy for the Devil," penned by creator Eric Kripke, where we actually get to hear some of that erotic fiction read aloud: "'This is wrong,' said Dean. 'Then I don't want to be right,' replied Sam, in a husky voice.'"
Kripke, who's now showrunner of Amazon's The Boys, recently touched upon his fondness for this type of fan adoration, confessing that while he hadn't read much of it, he was "flattered" by its existence all the same.
The crowning glory of "Wincest" content in Supernatural, however, is the show's 200th episode. Not only is it the show's first musical episode -- an odd fantasy television staple since Buffy the Vampire Slayer's "Once More With Feeling" -- but from its title, "Fan Fiction," to its content, it serves solely to reciprocate its adoring fandom's efforts by way of a high school play based on the "Supernatural" novel series: "Supernatural: The Musical." The fans take center stage while the Winchesters take a seat among us, the audience.
All of the parts are played by teenage girls -- an appropriate nod to the show's main demographic and a chance to hear some hilariously gruff-voiced acting. As well as "Wincest" references and Sam and Dean's continuing discomfort at the concept, "Destiel" (Dean/Castiel) and even the less-popular "Samstiel" (Sam/Castiel) ships are also given their due, while the show's queer female fanbase is actively acknowledged by the discovery that "Dean" and "Castiel" are also a real-life couple.
While the brothers never give "Wincest" their blessing (understandable) the episode uses the characters to directly, and poignantly, interact with Supernatural fan fiction, even taking a cue from it at the end. As "Sam" and "Dean" drive off into the sunset in their Impala, they tell each other, "It's the two of us against the world." Watching on from off-stage, Sam endorses the sentiment to Dean: "What she said."