brdgrl but presumably your dd is related to your step children so of course it is different.
wannabe, I'm talking about what people have actually said to me on MN. Those people don't see it as different, they still trot out the "they didn't choose...." line.
Of course many children are resentful of the arrival of a new bio sibling, hence the existence of sibling rivalry. But children don't seem to be permitted the same levels of sibling rivalry when the new "sibling" is a step sibling and is thrust into their lives when already a few years old.
I don't think that's true, actually. In my RL experience and my MN experience, most stepparents/parents are very conscious of the difficulties for the children, and considerable adjustment is made for that fact.
If the op's dd hadn't wanted to invite her biological sibling and had kicked off people would have assumed it was natural sibling rivalry, but she kicks off at a step sibling and is branded spiteful and all manner of other things
I would not be willing to dismiss the behaviour as an acceptable level of sibling rivalry. If the OP was about two sisters, I would not be willing to My own two DSC are close in age, and I would not see this behaviour as acceptable or treat it as 'oh well, sibling rivalry', as you suggest. If a child is being spiteful towards another child and kicking off (and here I am talking about the hypothetical child you propose, not about the child in the OP, who I myself have not referred to as spiteful, just to be clear!), that is an issue that I would not be content to write off as sibling rivalry.
I also think it is wrong to call it sibling rivalry, when the very point you are making is that they (the girls in this situation, at least) are not siblings and clearly don't live as such. The girl's behaviour needs to be considered not as that towards a sister, but as that towards a stepsiter, which often is a different relationship altogether.
and yet she's still expected to embrace this girl as a sibling, just as long as it's positively.
I don't disagree with you that there is a terrible double standard and a terrible attempt to demand that blended families 'have things both ways'. I often say so myself.
My point is that the line about "not choosing it" is used as often to insist on that same false positivity and double standard. For instance, in another thread, the OP could very very easily be told "You sound really unpleasant, with your criticisms of your DSD for her tantrums and misbehaviour. Of course she misbehaves [insert here: steals, hits your DD, refuses to speak to you, breaks your DD's toys, insults you, whatever] - after all, she didn't choose to be in a blended family!"
There are hundreds of examples of that phrase being similarly trotted out on these boards, and it's bullshit, every time.