Kiss x Sis can be described in pretty much a single sentence: a guy named Keita and his two step sisters Ako and Riko doing horny and perverted things to each other around the clock, solely because they want to. That's all. That's all it needs to be. What makes Kiss x Sis stand out is that for once it is actually hot. The problem with most ecchi anime is that their ecchiness generally only consists of random panty shots and breasts plastered across the screen, and I think most people will agree with me if I say that you need something a bit more sophisticated than that if you want to turn people on. But Kiss x Sis has almost none of that. Instead what the show does have is countless deep kissing scenes, licking and sucking of just about all body parts, grinding, masturbation scenes, on-screen orgasms, countless types of sexual plays and more. In other words it actually *does something* in a sexual sense, instead of solely trying to rely on eye candy. As a result, what you end up with is a non-hentai show which can legitimately be used as fap material without forcing yourself, and that's something extraordinarily rare to see. N-Not that I would do something like that myself or anything.
As far as ecchi elements themselves go, I think Kiss x Sis is most likely the best anime within the genre. There is no other anime which can consistently get you aroused as a viewer as much as this one does. The only thing it's lacking a bit is actual characterization, as there is very little information given about the girls themselves which make you not think of them all too much. All you'll really care about is seeing them get all hot and bothered with each other. Not that that's too big of a problem per se, but if the show had as likeable girls as other borderline hentai series like To LOVE-Ru Darkness for example, then I'd probably have to give this a 10, as in that case it'd be as close to perfect as an ecchi anime can get.
Another thing that ticks me off a bit is the side characters in the show, specifically Keita's homeroom teacher, her little sister as well as a random glasses girl from his school. While they're not bad characters per se, and while they also have really hot ecchi scenes with Keita, I can't help but feel like those scenes would have been even better had they simply starred Ako and Riko instead. The wincest feel is a major aspect of the show after all, and thus not using it to its full potential feels a bit wasted to me.
The soundtrack mostly consists of background music appropriate for sex scenes. Yes, it's just the kind of jazzy background music you'd find in porn films. On the other hand the opening theme is surprisingly awesome and super catchy. As far as voice acting goes, be prepared to hear a lot of moaning and dirty talk, but the seiyuus definitely pull it off with flying colors.
Overall there are definitely a few things here and there I can nitpick on but they're still quite minor stuff. What you see is still what you get, and Kiss x Sis stands out as quite likely the hottest non-hentai anime ever made, and that's a quite impressive achievement in and of itself. If you've only seen the TV airing of the show, then I'd recommend giving the OVA series a shot as well as they're significantly lewder and all-round better for its own intents and purposes.
If you're looking for a textbook showcasing of what really good ecchi looks like, then look no further than this. Just make sure that your parents aren't home when you decide to watch it.
