Kyoudai Shikkaku
One night, Yahiro (17) sees a video of his older brother (21) in a gay video and has a sleepless night. When he wakes up in the morning, his brother is there as usual, preparing breakfast and lunch for him. He can no longer look his brother in the eye due to the awkwardness, and so begins his days of anguish.
(Source: Hakusensha, translated)
Note: Includes 2 extra chapters.