it's reminiscent of two book plots, though, which is probably where it came from.

nobody was skeptical of the lannisters until jon arryn and stannis figured it out. maybe varys or LF - but we know what kind of plans Varys has in mind and it's not baratheon babies.

that baby would have been the heir so why steal that one and not the others? if it was jamie trying to promote his seed - he'd have just killed it considering what he did to Bran.

in the books there are other known bastards of Bobby B- and we see how the show handles any kind of subtlety so I doubt they'd have introduced this wrinkle on their own and then just let it hang a few seasons.