Hels61, you say the stepmother has "equal say/input in every minute detail of the children's lives".... can you give some examples. And also - how do you know what is going on at home at your ex husband's place, did your children tell you or did your ex husband?
I'm a stepmother myself and I would never get involved in every minute detail of my stepson's life. We have him 50% and I do the school runs, I cook him dinner, I wash his clothes, I help him with his homework, yes... all these things. But on the other hand I would not get involved when it comes to choosing his school, the courses he should attend, medical deicisons, how he spends his holidays, etc... as I consider that not my duty or responsibility.
I think a stepmother can only be the third parent when both parents authorise her to do so. So when my husband cannot look after SS he asks his mother first if she wants to take him. If she says she cannot or does not want to and I have to look after him - in that case I'll assume that she gives me third parent status and during this time I expect her to leave me alone and not hassle either me or SS with phone calls about "how we are doing". In her absense and my husband's absence I am authorised to take any decision for SS I find is right. But only then.
To parents evenings I would only go if neither she nor my husband can go. Same for school birthdays, doctor visits, etc. ... my husband sometimes asks me for advice re SS and in this case I'll give it to him. Or if I have the feeling something is wrong or could be done better... but apart from that, the responsibility is for me with the biological parents.
Having said that, my home is my home and I decide what rules my stepson follows. And if he transgresses rules, my husband disciplines him. I would never discipline him, but I will make sure my husband does on my behalf if there is some misbehaviour (which is rarely the case).
Apart from that I am his stepmother, I feel for him as if he were my own child, I spent the same amount of time with him than his mother does, through my presence, my opinions, my lifestyle I obviously have a huge influence on him. But from there making major decisions about my stepson's life - no.