Kiss x Sis is a pretty strange story to begin with. Essentially, the main male character, Keita, has two twin sisters that are not actually blood related to him. They want him in the worst way but he's very reluctant at first. As the series progresses, he starts to act a little mor impulsively as he tries to get into their high school so they can all be together.
The story isn't TOO bad but it doesn't really develop too much. There's nothing of any real consequence going on but that doesn't mean that it's not enjoyable. There is a great deal of sexual content that ALMOST reaches the level of an H-type anime. Almost.
The animation is actually quite good. It's not done half way and all the characters are given the same attention. It's not perfect since some of the backgrounds are pretty bare but overall it's nice.
Sound quality is excellent. It's clear without any noticable distortions or odd sounds. The soundtrack fits the anime well. One of the things that gets to me, however, is the over exaggerated sound effects you hear at times. An example would be the slurping of tea.
The three main characters are Keita, Ako-nee and Riko-nee. Development between the three happens pretty quickly, expecially since the series is only 12 episodes long. They blend well with each other and the secondary characters so you really don't see a large clash.
If you like this kind of anime, then it's worth seeing. If you're looking for an H-type anime, this isn't it. It's a fun series to watch while you're waiting for the one you really want to watch to download.
My overall score for the anime was calculated using a standard average calculation. ((Story + Animation + Sound + Characters)/4))