He doesn't have one. It was never mentioned in the manga or anime, and if he did, it would have come up eventually.
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He had at least five brothers. Four were killed in battle, when Madara was still a child. He also had a younger brother, Izuna Uchiha.
No, Obito and Madara have no relationship or connection with each other. Izuna is Madara's brother whom he takes his Mangekyou Sharingan from to gain his Eternal Mangekyoi Sharingan.
Brothers, The Brothers Grimm, The Blues Brothers, Four Brothers, Step Brothers, Seven Brides for Seven Brothers, The Brothers McMullen, The Brothers Solomon, The Brothers Karamazov, The Brothers Bloom, The Corsican Brothers, ...
Because Tobi is from First hokage generation and Obito was from Kakashi generation.
Obito dies in the series Naruto