Watashi ga Aishita Onii-sama

The protagonist wakes up one morning to find himself turned into a tentacle monster!
His parents don't recognize him and try to get rid of him; however, his 2 younger sisters sense that it is their brother.
From that day, he starts living in a closed storage.
Yet, he enjoys a sex life with his sisters while making use of his tentacles.

[From DLsite]