Upon the discovery that the woman impregnated by Satan was going to have twins, it was believed that Satan's powers would be split between them. However, because Yukio's body was weaker compared to Rin’s, Yukio did not inherit any of Satan's powers and was born fully human, giving all of Satan's powers to Rin. Yukio did receive a spirit wound at birth from his brother, which allowed him to be aware of demons for as long as he can remember.
Poor Rin must have fallen asleep from studying manga so hard. I think Yukio will let him off the hook this time. Thanks to Chris for sending this and to さむきえるけー for sharing it!