Wincest fuck or die / love spell witch case fic

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Hi, I am looking for a fic I read a few years ago! IIRC, Sam and Dean are investigating a case in a town where unlikely couples are suddenly getting married because of some kind of love spell. At some point Sam and Dean get hit by the curse and have to fuck or die, and they give into their growing feelings and have sex. Afterwards they still feel attracted to each other and they assume that the spell hasn't broken but it turns out they are genuinely attracted to each other now. I think the witch who cast the spell makes fun of them for it? 

 I'm very hazy on the details so if this sounds at all like any fic you've read please let me know. Thanks!