N. K. Jemisin gives us enough information for a definitive answer. Right after the climactic event in The Fifth Season we see
Her fellow survivors will find her and take her to the mainland. There she will wander, lost and losing herself, for two long years.
If it is not Jija, it's also not Alabaster or Innon. Connect this with the beginning, where we see
You're an orogene who's been living in the little nothing town of Tirimo for ten years...Jija's a man Tirimo born and bred, a stoneknapper of the Resistant use-caste...
And then, at the very beginning of The Obelisk Gate,
Nassun next, then. Nassun who is just eight years old when the world ends.
Had Nassun been someone else's child than Jija's, Essun would have had to have had an affair with some unknown man in the little town of Tirimo, and this never appears in the trilogy at all.
N. K. Jemisin is too good an author to insert a pair of unfired Chekhov's pistols like that.
Also note that Nassun has (had) a little brother, Uche, who might have been even a stronger orogene than her.
Essun's character growth in the story is driven by her decision to stop letting things happen to her, and do things herself.
But this really begins only after she finds Uche's body.