If you're looking for manga similar to Yosuga no Sora, you might like these titles. All recommendations are made by Anime-Planet users like you!
Aki lives a charmed life. Popular, beautiful, athletic, and at the top of her class, she rules the school, and at home, her younger brother Sora caters to her every whim. Her younger sister Nami, however doesn't have it so easy. The small-chested and sarcastic girl is in love with her best friend - who has a crush on Nami's twin brother Sora - but her life could be worse: she could be her brother. Slight, feminine, and bullied by his twin, Sora cooks and cleans for the entire family. In addition, he harbors a deep-seated lust for his older sister, Aki. Will Sora get over his fantasies or give in to his desires? Will Nami ever win the heart of her best friend? And what does Aki want out of all of this?
Sibiling love is forbidden... or not? I can't go completely fanboy like in the case of Aki Sora <--> Kiss x Sis, but this recommendation works fine too. Both Aki Sora and Yosuga no Sora tend to be more of a dramatic type of manga, with all the thoughts and the doubts behind the sibiling love (which is fine in manga and anime *wink wink*).
Aki Sora's romance is primarly incestuous. The harem you're going to find in Yosuga no Sora is not, that's the real difference.