As can be observed from the title, the central taboo is the incestual angle of Keita and his two loving sibling-in-laws Ako and Riko. The series begins with Keita weakly protesting the sexual advances of his step-sisters, only to routinely cave at each and every opportunity. This progresses onto several other girls throughout the plot as he establishes a harem that increasingly craves the body of Keita, and he responds appropriately by becoming more and more assertive in responding to the advances of his admirers.
Typically, the sex appeal and likability of the admirers for the lucky protagonist are what make or break the appeal of a harem manga. But I posit that it is Keita who made this manga such a successful lewd romp. In Keita do you have a perpetually horny main character who instead of shying away from his admirers, embraced his libido and acted upon his urges at almost every opportunity. The man has no restraint whatsoever and can only be stopped by the divine intervention of Ditama Bow's penchant for ill timed interruptions. He almost completely embodies the will of the reader as he entertains every single member of his harem at various points of the story. While Riko and Ako early on do get the lion share of the attention, there is actually a point where their appearances become fairly sparse and other girls take the spotlight, which truly helps to keep the fanservice of this story fresh and creative.
This ironically leads to the biggest criticism of KxS however - for a series that succeeds so well at entertaining a wide net of perverted antics, Kiss x Sis never pushed its degeneracy far enough to fully climax reader satisfaction. Case and point - not once was there full nudity of the characters involved. While Keita, the main character, can completely see the exposed breasts and genitals of his harem the reader is always left using their imagination to fill in the blanks as their bodies are obscured. This creates a disconnect between the reader and their intended self insert hero as he's always the more privileged observer, which at the very least the ecchi reader asks for is to share their vision when to engage is never a reality.
Another, but far less important, criticism is the lack of a romantic commitment in this story. Kiss x Sis is better described as an ecchi comedy, with a hint of romance, than an ecchi romantic comedy. So if you are someone that gets personally attached to your waifus, ride or die for your waifus, are willing to catch a life sentence for your waifus, then you may want to brace yourself for the disappointment of your waifu not being placed on a pedestal above others and being “chosen”, as this manga is far more concerned about entertaining all parties than prioritizing any one. A lot of veteran Kiss x Sis readers were disappointed by this turn out, so try not to be one of us.
Needless to say, this isn’t in any way some thematically rich or high brow story. Much of the content is episodic and the tone vacillates between extremes of silliness and passionate degeneracy, with little to no in between. If that sounds like something you’d be into however, then this is a must read as there have been very few ecchi that blend lewdness and comedy together quite like Kiss x Sis. I will miss this manga and the niche that it filled for such a long period of time.
