I'm not talking about the stepchildren, I'm talking about the OP's children. If the OP's young children are having rubbish Christmases because of this bonkers arrangement, that's not going to change at the request of teenage stepsiblings, is it?
The problem here is obvious. Lunchtime is a completely antisocial time to do the handover. If they insist on splitting Christmas Day, either the OP's husband needs to pick his kids up slightly earlier so they come over for lunch (and don't eat Christmas lunch beforehand) or he needs to pick them up later in the afternoon so the OP and her husband and children can have their Christmas lunch first.
And her husband's ex wife should be doing her fair share of the driving.
It's utterly shit for the OP and her children to be abandoned over lunchtime every year (on strict instructions not to go to the OP's family for lunch either) and for the OP to then spend her afternoon slaving away making an evening meal that her stepchildren don't want to eat because they had their big meal at lunchtime. That would be shit any particular year but making them all do that every year until the stepchildren have grown up is ludicrous.
The stepchildren might not have chosen for their parents to separate. The OP and her husband might have chosen to form a relationship knowing that he had children from his first marriage. But that doesn't mean that this arrangement needs to be preserved for all eternity just because his children from his first marriage are innocent in all this. Other people are innocent in all this too, namely the OP's children and her parents.
As for all the people criticising the OP's parents for not eating their Christmas lunch in the middle of the afternoon so the OP's stepchildren can attend, what on earth makes anyone think her stepchildren want to attend?
Imagine you are one of the OP's stepchildren and your Christmas Day looks like this. You wake up in your mum's house, open your presents while your mum is cooking, gobble down your Christmas lunch at 12 o'clock when you're probably not even that hungry but you have to be ready to get picked up by your dad at 1 o'clock, then instead of being able to relax around the table or get back to your presents, you have to get in the car and drive to your stepmum's parents' house, i.e. two old people who aren't even your own grandparents, where you're expected to sit politely at the table for a second Christmas lunch about two hours after your first one, which obviously you're too full to touch. And you don't really get to spend any quality time with your dad or look at your presents until much later. How on earth is that a good Christmas Day for the stepchildren?
It makes so much more sense for them to stay with their mum for a few more hours. I expect she would prefer that too.
So who is the current arrangement actually benefiting?
No one.