You can certainly do spellwork on his behalf without him even knowing of it. Actually, i would usually suggest this.
I'd recommend work that is two-part. One, perhaps look into a freezer spell to render your mother harmless. These are pretty easy to do and require little ingredients. There are many variations, so i'd research one you ressonate with. Two, i'd look into doing protection and healing work on your stepfather's behalf. Again, you have many, many options.