Board Thread:Theories and Speculation/@comment-28939725-20161121090907/@comment-25122992-20161121092231

I think if Obito was really his brother, it would've been clearly stated in the series, at least one or two times. Also, Hiruzen mentioned only Shisui while speaking about Kagami's descendants inheriting the Will of Fire, but Obito also seemed to be its believer and "died" sacrifising himself for his friends, so if he was also from the same family, he would've been mentioned along with Shisui as well.