Hold Me Fawn On Me chapters 1-2 [English], by Soba (Also going by the name Saz, not to confuse with another artist called Kamoro-Saz)

Take the traditional delusional younger sister (in love with her brother and having huge fantasies to the point of distorting in a funny manner her perception of external reality).
Give her a rival, a karate practicioner with smaller breasts.
And finally, give up on any pretence of making an interesting story, as long as you’ve got a basic structure you can throw as much sex as you can.

– That was today’s hentai recipe, with thanks to Raikoh from Doujin-Moe.us