Gimainara Honki ni Nattemo ii yone?

Masato Kyoki never got along with his own sister, Yurin. However, when their parents suddenly reveal that they aren't actually blood-related, Yurin is so shocked that she loses her memories of Masato. Masato is concerned about Yurin's mental state after she is released from the hospital, but for some reason their relationship is now much more intimate than before!　Yurin seems to be coming on to him, which is the complete opposite of her attitude before she lost her memory, and he can't help but get all flustered!

(Source: HJ Bunko, translated)