Most likely, the stepfather's suspicion is too high. You can either lower it by giving him beer from 9:00 PM to 12:00 AM.
My tip for either step-father and step-bro route is finish them as quickly as possible. It is the most punishing route with the suspicion gain passively (especially when they hate you).
To reach a good ending with them, you need their:
- Lust: 50
- Affection: 0 or 45
There should be an option of being theirs forever in the end.
Another choice is to get to the throuple phase with them, you need their:
- Lust: 50
- Affection: 45
See the step-dad and choose the option "He has some serious things to talk to you".
Hope this helps!