I have no idea why and it may be entirely unreasonable of me to feel like this, but step-parents going to parents evening really annoys me. I would qualify this by adding that if the step-parent is a resident step-parent and the other NRP parent has little or no involvement in their child's life then of course it would be more than reasonable for the step-parent to go to parents evening.
However in a situation where the step-parent is living with the NRP and maybe sees their step-child once a week then I can see no good reason for them to be involved in parents evening.
Surely there has to be some instances where, if there are two involved parents, only the actual parents need to be present, even if two separate appointments are required.
In my particular case I do not go to my DSDs parents evening and I would not expect to go. It is for her parents to be interested in how she is getting on at school and my interest is limited to asking her how she is enjoying it and praising her when she gets a good report etc. By the same token my DH does not go to my DSs parents evening, even though DS lives in the same house as us. She spends two nights a week at our house and does quite a bit of her homework when she is with us.
However my exh takes his partner to parents evening, even though over the last eight years my DS has seen his dad less and less, due to her influence and his disinterest in actually seeing his son. He now has DS overnight twice a month and thinks that he is doing me some sort of favour by doing this. He told my DS that he needs a social life with his partner and that is why he no longer has him overnight once a week.
We have all come to terms with my ex husband's lack of involvement in his son's life, however it does make me a bit cross that he insists on taking his partner to parents evening even though she sees DS twice a month for about 12 hours at a time.