Wait isn't this Manga technically Incest lol

[Pumpkin Ninja]

The relation is so f*cking small, it would be insane to worry about that for them.

 

[Brady]

Where is stated that Mikoto is an Uchiha by birth? She can easily be of another clan, which is more likely.

I think she's an Uchiha. She's married to the head of the clan, looks like an Uchiha, and had an involvement with the coup. On top of that, I'm sure that the children of the head would need to have strong blood unlike their more distant cousins.

Considering the tensions between the village and the clan, I doubt they'd let some woman waltz in and marry the head, being of another clan. If Sasuke had heritage from two different clans, it would have been covered.

Obito is more likely to be half Uchiha than Sasuke or Itachi, but considering how large the Uchiha clan was at it's downfall, Fugaku finding a pure blooded woman of distant relation would have been easy.