The problem with this is that Rachel is an essential character to the plot. Without her moving into the house, the recovery of the relationship between the MC and Ellie would be impossible, not to mention everything else. How essential Rachel is for the plot is also made clear by the fact that she will get her own little story after the main story is finished, I guess somewhere between the escape in the night and fog from the MC and the MC's moving back in. Theoretically, the character could probably legally insist on his stepmother moving out, but that would only mean more trauma for Ellie and would be the last thing you want to achieve. I think recreation will leave it up to us as players whether we pursue any sort of romantic relationship with her. As for Rachel, I think she's screwed up a lot in the past, but MC and Ellie's father is the main culprit. She should have sought professional help much sooner after his death, but how many people with alcoholism and the other stuff that burdened her, do? So I think MC's escape and what happened to Ellie was the wake up call she needed. For many, even that wouldn't help anymore so I don't think she is a bad person in that sense but rather a tragic figure who became the victim of circumstance.
Accordingly, I can also understand the MC being rather skeptical of her, after all he couldn't help what happened to her, but at the same time I think the MC as a character needs to get over the past, especially for Ellie.