OP had said she will get cakes and have a little celebration when the step kids are next with them.
This isn't the party though is it...I mean this could happen any old day they're there...it's all the other people who would be there...it's hardly a party with your siblings alone .
one set of children are set above the other.
Not at all
Which, as you say, is fine for a toddler who doesn't know the difference,
Exactly... and as they get older, you change things.
but kids grow and I can think of more than one thread where the SM is in despair because her DH won't so much as order in fish and chips when his elder children aren't present
This is foolishness obviously and what stops her ordering the fish and chips herself. If he doesn't want to eat them..it's his lookout.
What I'm pointing out here is perspectives. Her husband wants ALL his kids there...she has ALL her kids there ...so it's not an issue for her, but that shouldn't mean she shouldn't take a minute to understand how HE feels.
Perhaps he should also reflect the facts that she isn't bothered because his kids not being there doesn't matter to her, while it matters to him.
The stepfamily dynamic takes understanding on both parts...it doesn't necessarily mean one person is right and the other is wrong.
They can both be right and justified from their own perspectives. I'm just saying they I understand his perspective.