There isn't any reason for your assumption. It seems that you just picked up some character who should occupy Tobis role. Which reasons can you give me for your assumption?
Well ok I'll go through it.
Wew why tobi could be the younger bro.
1. kishi likes twists
2.obit is able to use advance eye techs without strain on chakra a feat that even many uchiha would find incredible.
3. His hair is similar to the younger brothers
4. The myth states that the younger brother was attacked by the older brother but does not go further into what happened, in that battle.
5.Tobi is the master mind behind akutsuki, meaning he located the statue or already new where it was, and created the sealing methods for the juubi.
Why tobi could be Older brother.
1. Tobi's mask is a spiral
2. His eye techs are unique to even madara's
3.He has the same goal as madara and is atleast a peer with madara
without being mentioned in nin history.
4. Tobi is the master mind behind akutsuki, meaning he located the statue or already new where it was, and created the sealing methods for the juubi.
Why Tobi could be an uzumaki
1. Uzumaki specialise in sealing techs, tobi easily seals other nin
2. After the great war the uzumaki clan dispersed all over the world, main clans that faced destruction after the great war have seeked revenge but not the uzumaki clan yet.
3. Tobi is old, uzumaki allow for longevity, this would allow tobi to have been alive around the time of the great war.
4. Tobi possibly wants to use the special uzumaki chakra to seal the bijuu within himself.