Well done for doing this now.
I feel that if a step child lives with you from very early childhood, then that is different. Since your relationship with your wife started with her daughter at university, she was an adult.
The most harmonious solution that I have seen was with my god mother who had a 2nd later marriage with a man where it was also his 2nd marriage. They each had adult children. Their wills were very simple in that his children inherited all his assets and her children inherited all hers. It was very amicable and no-one was upset.
I grew up too with a friend whose parents had married, both with toddlers and alot of money on both sides. The children were brought up equally but inheritance went to the natural, blood children. again, everyone has been happy.
I hope that helps.