I, for one, am not yet convinced of the purity of its intentions. The girl has only interest in sis con eroge, not yaoi or other shoujo shipping fodder. That’s weird no matter how you cut it. Then, she’s a pro model in middle school(?), without her brother or parent’s knowledge. I’m not saying it couldn’t happen, its the way the episode brushed it off like it was nothing was what set off alarm bells. And finally, like you noted, it’s preoccupied with framing the girl in compromising positions, like on top of her brother in bed late at night. It’s saying one thing, but doing another, which makes me believe its intentions aren’t as kosher as it seems.
Kadian and others point out the use of risque scenes in Ore no Imouto, which raises an interesting question: such scenes appear throughout other anime. Are we being hypersensitive about them here because of the undercurrent of normalizing incest?
If we agree that the show doesn’t make a big deal of it, what then? Do we actually think that is going to cause something to happen in real life? (Do violent video games cause violence? Do rape eroge cause rape?) Or are we simply reflexively shuddering at the thought because we are trained to do so?
Questioning society’s taboos is the most essential function of art, so even if the show is pandering and risque, in this context, it would be in some sense artistic. In this case, it’s also questioning some of anime’s conventions as well. Sure, the sister is framed in incredibly weird positions. But when that sort of thing occurs in other shows, our normal reaction is to laugh.
Are we so dead set on our taboos that we can’t even discuss them?