Watashi wa Oniichan to Tsukiaitai.

Original doujin manga, 2nd of the series. It all started with a kiss, and a brother and sister both became hooked on an incestuous relationship.

During summer vacation the brother, who now lives by himself, gets visited by his little sister who came to stay at his place while keeping it a secret from their parents. Of course, there's no way nothing will happen between them... it's that kind of story.

While this is a sequel, it can also be enjoyed as a standalone work.

(Source: DLsite)