I could see Wincest growing out from the SPN backstory because the brothers have such an isolated relationship. Their links to community and family (aside from one another) are pretty nonexistent, and they're both so broken and dependent. I can buy stories that show that sort of metastasizing into a sexual relationship. Ending up as 80+ year olds in rocking chairs in Ye Olde Hunters Home holding hands, not so much. Then again, I don't see either of them seeing 80 no matter who they have sex with.
But this sort of runs aground for me when I watch the show and fail to see a sexual spark. Their on-screen relationship reads brotherly to me. I felt there was occasionally a vibe between Simon and River, and for that matter the Lex/Lionel sparkage was there if I squinted. But I just don't see it with the Winchesters.
This doesn't stop me from reading it if an author I like goes there. But I don't watch the show and get too OTPish about the boys. Dean/Impala, maybe.