First I have to take my hat off to you for
A) researching and doing what you can to be the best parent you can be and
B) stepping in as role of mother to this child
if she hasn’t seen her biological mother since she was a baby, then you and her father ARE her parental guidance… you should raise her as if she were your own biological child, with the same rules that you will have for your kid(s)
if that involves keeping schedules, and that works well for you (I think schedules are important too!) then you should do your best to stick with a schedule for your step-daughter.