I can speak from a step mum view.
I have three of my own children and one step son who lives with me full time, he even calls me mummy, his mum have never had much involvement in him and he never stays over at hers, only sees her for a few hours once a week.
He is only almost 3 so not at school, when he does go to school it will be me and my partner who will go to parents evenings, although we wont say for instance she CANT go maybe to arrange another day for her, or take back a written report etc.
Im sure if you get on fine with the 'real mum' then you can work something out that is fair to you all, i thinks its nice that you take a good interest in your step daughter.
Speaking on another view, my ex now rings school up and speaks to receptionist about any reports they do two copies one for me and one for him, we take it in turns to gothe parents evening thing.