The Sage's 2 sons were neither Uchiha or Senju...

[Honord Sage]

Evidence their were No Senju nor Uchiha when the SO6P was alive,the Sons belong to His Father's Clan. At some point and time the descendants of both sons began calling themselves Uchiha and Senju to each other and the world, each stating their differences from one a noter and The SO6P.

 

[Sarahmint]

Guys, don't be so rude. It makes sense. The Clan descendants through storytelling were probably given that version of the story to make sense of what was happening at the time.

 

[klikzord]

People are always forgetting about theirs wives.

Senju origin = younger son + his wife ( probably Uzumaki), i mean his personality is similar to his father( The Sage). We have another example Hashirama + Uzumaki Wife.

Uchica Origin = Elder son + his wife ( probably Hyuuga), fetish with eyes, and explains why they are distant relatives.

 

[RikudoKami]

Sage probably had two different wives