It's far, far too varied for a blanket yes or no though.
I judge any adult who is complicit in making a young child feel miserable and second-best
Not going for a sleepover or receiving huge amounts of presents from my parents (who they barely know) would not make my DSC feel this way. They are older kids, teenagers now, they understand the reasons perfectly well. Statements like this add emotion which may very well not even be there in a lot of cases. It's not about being second class in our situation, they just aren't their grandparents and that's fine, they get it, they don't need spoiling with lavish gifts from 4 different sets of 'GPs' in the interest of making everything completely equal or having offers of sleepovers forced on them that they'd likely find awkward more than anything else. I'm not sure what they'd even find fun about a sleepover with such an age gap anyway.
I would probably arrange days out/sleepovers with DD for when the step kids aren't at yours, so no one feels left out
I can understand this in some cases (not mine, my SC wouldn't care) from one angle but on another, I think my DSC would quite like having Dad to themselves for the night/day. It's rare that they get that now they have a half sibling. If my mum coming over and taking out our joint DC (who is much younger so unlikely to do anything they'd like anyway) means they get to spend some alone time with their Dad, I actually think that's a good thing. They are there to see him after all, not my parents.
If children are going to be forced to spend substantial amounts of time (weekends, Christmases etc.) with "family" who don't view them as family, that's not OK. If actually, the children only visit EOW and don't have to see the step-grandparents that much, then it's probably fine for them to be treated differently
It's not just EOW type set ups. We don't force anyone to spend time with anyone all the time and our DSC live with us 50:50. We spend Christmas together just us. My parents don't come round all the time during a normal week so they still don't see them a great deal. Occasionally they come round for tea and are nice and pleasant to all children, the other times it's usually me taking our DC to them on a weekend or something.
I personally think it's us adults that spend far too much time going round in circles getting tied in knots trying to make everything equal all the time when it doesn't need to be. Equal is not the same as fair.
Why would it be fair for, for example, my DSC to go and be spoilt by their GPs on Christmas on mum's side, come to ours and tell us all about it, what they got, what a lovely time they had, brag about their presents (because kids do) but for my DC not to get anything that they don't also get from my parents? Funnily no one cares when it's that way round though. Some children really are old enough to be expected to understand this imo. I'm not talking little tiny children or children who've grown up thinking of these people as their grandpar, but older children, I don't see why they can't be expected to understand that they get X from their GPs and their sibling gets Y from theirs.
Same with inheritance. I actually don't find it that shocking that someone would skip a generation to ensure their grandchildren inherited. People want to ensure that their family (and no, if you are barely involved with SC and they don't have that grandparental relationship with you, you're unlikely to see them as family in the same sense), get something from them and not have them diminished by other parties further down the line. Don't see what's so terrible personally.