User blog comment:Omega natsu2/Chapter 364: Tartarus Arc, Part 1: Immorality and Sinners, Predictions./@comment-7418318-20131211093800/@comment-7418318-20131211174436

Yes. If an Uchiha uses his own Mangekyou Sharingan, they'll sooner or later go blind and lose their powers. But if they use a sibling's Sharingan, that won't happen.

And killing your own sibling is also one way to obtain the Mangekyou, so killing your own brother and stealing his eyes serves a double purpose.