Everybody is taking a guess but the meme really came from a legal situation (i forget which state) where incest could not be portrayed in porn but was precisely defined as a blood relative. By mentioning the male was a "step brother" somewhere it ensured the video could not be classified as incest in legal terms (while still appealing to....that crowd). It was used enough combined with awkward acting that it became a meme in general... And the Internet ran with it like it does. So it just sorta implies a porn situation now