Normally I wouldn't bother watching an anime with "incest" as one of the tags but after enjoying Kiss x Siss I thought I'd give this a try. Lets see... Where to begin?
The show starts out with Haru and Sora coming to live at their deceased grandfather's house in the country after their parents are killed in a car accident. Sora is haughty and petulant whereas Haru is the perfect gentleman.
The rest of the supporting cast is introduced and they actually were appealing and had depth(at least for an ecchi anime). With this being ecchi after the back story has been taken care of and all the characters were introduced the story branches off into several independant arcs that detail Haru seducing the three girls in his group of freinds at school and eventually having sex with all of them.
The last girl Haru has a relationship with is is neighbor Nao. Sora has been sexually attracted to Haru the whole time and of course she walks in on Haru and Nao having sex so she freaks out and eventually drives them apart. Eventually she winds up seducing her brother and it just gets weirder from there.
Whereas Kiss x Siss lightens up the taboo of sibling relationships with comedy and never actually crosses the line, Kosuga no Sora basically hits you over the head with full on incest action. I would have been fine if Sora was portrayed as a crazy sister that went a little nuts during those long years in the hospital and had a platonic love affari with Haru but they attempted to legitimize the relationship and that put me off.
Bottom line: There is a lot of ecchi anime out there and most of it is better than this unless incest is your fetish.