Board Thread:Theories and Speculation/@comment-28939725-20161121090907/@comment-3965545-20161121093059

Shisui's direct family was never talked about. But Obito was an orphan like Naruto in the sense that both of his parents died when he was really young. If Shisui was his sibling he would definitely know since they were both living with the Uchiha and Shisui should only be around 4-8 years older than Itachi who is 9 years younger than Obito, hence they should also know each other... maybe they are direct cousins instead..