The biological father of DJ is yet to be uncovered. Whomever this person is, either doesn't know it himself or wasn't ready for the world to find out about the news. What if he's been employing threatening or blackmailing of some sort to keep Erin from disclosing the truth, and when she resisted, in a fit of rage, killed her?
Also, the pendant Mare found in Erin's bedroom with the date "May 29, 2017" carved on it could lead to the father. Now I don't know the exact timeline of the death of Kevin (Mare's son), but could he be the father? (Not the killer, obviously)