It is really, really hard, and I am so sorry your dad, your stepmother, and you all have to face this.
No matter how much you want to keep them at home and they want to stay at home there just comes a point where it's impossible. You would never believe how much care and attention and work one other person requires until you go through this. And a lot of time the dementia means people really get going after four in the afternoon and all night--sundowning--which means the caregiver can't get any decent rest, downtime, or sleep when they most need it.
There are still good care facilities to be found, and if your stepmother can find one, she will be astonished at how wonderful some caregivers can be. I know I have been.
I wish you all the best of luck.