Thst sounds a lot like people pleading to me and op has described a one sided dynamic, that has been raised directly with them many times.
Of course all relationships look different, but both sides have to invest time, energy and effort into the relationship not just side, otherwise if breeds anger and resentment.
Given the distinct lack of effort made by them why should op and her dh go? Just to play a role and people please? It’s going to cost them nearly £1000!
It is common and likely that dh has been the black sheep all along and has had to live in this dynamic for his whole life, that is what op is saying if you read all of her posts. Dh has tried to address the issues with them - they have ignored him, and that is disrespectful.
No doubt dh feels sidelined and ignored, hurt by their lack of interest and effort in his family.
It would be best if op and her dh make much less effort. Cut back on the visits, stop paying for them to visit. Take the children on a proper holiday, and stop investing in people that don’t care about them.
Thar way they can maintain a distant, cordial fairly balanced relationship that is not harmful and painful, and full of resentment. As you said all relationships look different, and maybe it’s time to accept that this relationship doesn’t have much depth, love or richness despite the op’s best efforts.
Matching their energy and effort is the best way forward.