"Imouto Please" by Ayana Taketatsu
Hmmmm she sounds like Azusa in this song. Somehow I prefer the version of Hatsune Miku maybe because it's a lot nicer to the ears or maybe I'm just biased towards vocaloids XD. Anyway it's quite a catchy song maybe it'll grow on me eventually.
So apparently the real ED of Ore no Imouto was being streamed in the official website instead of the usual ED maybe because of time constraints so maybe eventually it'll be properly aired but if not maybe their saving it to the DVD/Blu-Ray release. Also I was surprised to find out that the original version of the song was sung by Vocaloid Hatsune Miku and since that song was quiet a hit in Nico Nico Douga, they made it as an ending of the anime Ore no Imouto and Ayana Taketatsu (Azusa, K-ON!) sang it. I just hope I could find a sub of the song somewhere.
Here's the Hatsune Miku version..
*Tnx to Nyarth for the news. Found it on his/her site :D