Queen's Seed

Youto has followed his childhood friend Mikoto, a girl one year older than him, to the same high school. When they leave school together for the first time in a year, he discovers that she sees him only as a little brother. Yuto felt a little dizzy at the thought of being rejected, but then an alien "onee-san" appears in front of them, dressed in a very revealing costume...? Youto is beset by all sorts of alien women in this "onee-shota" space opera!

(Source: Comic Days, translated)