My speculation... and rants(ignore it...)
So what I get from SnK chapter 65:
1. Levi is Kenny’s nephew
2. With kenny saying “ that one branch family”, Mikasa is from branch family while Levi from main.
3. and most important thing, Levi and Mikasa are NOT related directly
I can’t say much because I don’t know how Ackerman family works, if it’s the same as Hyuuga clan from Naruto (main house from older brother, while branch from younger brother) but I can say that Mikasa’s father is not an important person among Ackermans with Kenny gave so little though concerning his family. I think Levi and Mikasa are distant cousin or worse their connection farther than distant cousin
so that conclude why Levi and Mikasa pairing is not Incest, and for God’s sake, stop saying our ship has sunk!!!