Some people are very quick to Judge ! None of you LOT have a clue what the OPs parents situation is. As far as anyone knows there could be an extremely good reason for 'a trip of a lifetime ' with their grandchildren. As OP says, they can't pay up front but can pay the loan so maybe there is a time constraint like terminal illness that makes 'saving up' impossible.
I also don't agree that sc HAVE to be invited on ALL things ALL the time in this desperate bid to try and persuade everyone that life has to be equal at all times because their parents aren't together. ! Parents and Step parents tie themselves up in knots trying to make it all 'equal' when all you should aim for is 'fair'.
Look at this practically. This is not the parents holiday. This is the grandparents who want (for whatever reason - not our business) to take their dcs, spouses and GC abroad for a holiday. They are getting a loan so need to keep prices down. They do not have a relationship with DGSS , it doesn't matter why, they just don't, sometimes it's simply logistics and no ones fault. Distance between gps and OPs home, frequency of GPS visits coinciding with dss (who cancels often) and I would doubt he would travelling with his two pre school half siblings to visit them anyway. (Why would he want to) I expect he has no interest in your parents as he has gps of his own... so on that basis I think your DP is being very unfair to you and your dds. He is as MUCH a DF to his children with you as to his son. Just because he isn't invited on a holiday by someone he has no relationship with, nor by the sounds of it has ever had interest in, should your daughters be denied a holiday with their father.
You should all go. DS wouldn't be allowed anyway and certainly wouldn't enjoy the pre school dynamic aged 12 plus it would be awkward if he doesn't really know your parents.
It is your DH job to save up and take him away by himself, doing something age appropriate for a 12 yr old. Not enduring hanging out with toddlers .
I have children, step children who are both resident and NR as well as being a stepchild, step sibling and half sibling. It's hard, just stop trying to make it equal. It's not and never will be. Just be kind and thoughtful.