I've been trying to find a story (one in particular, though I'd be happy to discover any other of the same plot, more or less) in which a young man got back from his first year at university, and gets seduced by his sister (in the version I remember it is his actual sister, not stepsister or anything, but I'm not entirely obsessed with that needing to be the case). While he was at University for most of a year, it seems, she has gained 100lbs of fat, becoming a BBW, and it happened because their mom wanted her daughter to sleep with her son, and after he left, she'd discovered his computer full of porn of fat feedees who promised to get fatter for the viewer. So, she spent the last year getting his sister, (I think she might be his older sister by a year or maybe two,) who had stayed at home, interested in seducing her brother, and she helped make sure he's okay with it by helping her to gain a hundred pounds, so that by his return, he'd be seduced by a 250lbs BBW promising to gain hundreds more pounds of pure blubber and already almost unrecognisable as his skinny sister.