I'm trying to write this as dispassionately as possible, but apologies if I'm going beyond advice.
I'm like your step daughter, but my parents divorced when I was very young and both remarried before I was five so my step-parents have been there for all of my retained memory. Following my mum's second divorce (25 years later), my now ex(?)-step dad is remarried. Just to add in the mix, my wife's parents also separated and then divorced when she was in her 20s.
All parents have got the age where they're planning wills. My step-parents' attitude couldn't be more different, with one wanting to give almost every thing to their child and the other splitting everything evenly between a number of children. Knowing this, certainly has an impact on my relationship with my parent in that relationship.
You mention wanting a more harmonious home life. I think you've got to ask yourself the question of what relationship you'd want with your step-daughter if her mother were to pre-decease you and indeed how you want your wife's relationship with both of her daughters to be while you're both still alive. Also, who would look after your daughter if both you and your wife died at the same time? I'd definitely be open with the adult daughter as to what your decision is.
You don't need to split the estate evenly, but providing nothing for your step-daughter (and with the best will in the world, who did exist and presumably was part of your decision when marrying her mother, despite her being an adult) seems (in my view) unnecessary. Bottom line, what would you both say to your step-daughter to have her understand your joint decision, and for her not to feel hurt by it?