Nobody can make you drink but you. Your stepmother can be the worst person on earth, but you can decide how you will deal with her. Don't let her give you a reason to drink. Nothing matters more than your sobriety.
In between visits with your father/stepmother, do something relaxing and fun. Treat yourself. Go shopping, see a movie, something you enjoy. And if you are tempted to drink at any time:
1. Remember your last drunk.
2. Remember your last hangover.
3. Remember the despair, hopelessness, frustration and self loathing.
4. Remember how hard it was when you first quit and imagine what it would be like to have to do that again ... if you were even ABLE to quit again.
5. Compare your new life of sobriety to your old life of drinking and ask yourself which one feels better.
6. Repeat 1-5.
I think you will do just fine; you're already well aware of how this woman makes you feel, so arm yourself in advance with a plan and be ready for her. Don't let her win.