I am your DIL in this scenario.
My MIL asks my DH every now and then if we want to go away with them. To be fair, he is pretty bad at replying to texts (not just to his parents), but he always politely declines - even if not always in timely fashion.
FIL made comments to my DH a few times if it's me who doesn't want to go.
In reality, my DH doesn't want to spend holidays with his parents (neither do I) as we like different type of holidays and we don't get on with them much. We stay at theirs 2 per year and it's always very tiring and we come home exhausted.
I think you are blaming your DIL here, but are you sure your DS would want to holiday with you?
Not everyone enjoys going on holidays with their parents / ILs.
Just because you enjoyed it, doesn't mean that they want to.
They should reply to you though and decline politely. But again, this is up to your son not DIL.
I would not push the issue, they clearly don't want to go away with you, so the more you push, the more awkward it will get.