TM, it's sad that your in-laws are like this. But hey, some people are just jerks. Even those who should actually exert themselves and make an effort - they are still jerks.
Kids need to learn that adults around them are not perfect, that there are grades of people who can be relied on, through to those who are a waste of space. She needs to learn that even people we love, who say they love us, can be a disappointment sometimes.
It's a nasty lesson, one that is best learned gently and slowly, while you're young but also while you can be cushioned at least a little bit.
In an ideal world, kids should never have to experience this. But people are basically selfish. Some of us are better able to actively work to overcome this basic innate selfishness tan others, that's all.
The end result - Duckie will learn, sooner rather than later, that that particular set of grandparents can't be relied on. The earlier she learns it, the earlier she will learn to extend this experience to develop a thicker skin over it as well as a thicker skin against the same thing in other people in her world. It will help her cope better with the world's disappointments in general.
And very soon, if not already, Duckie will have a sound, solid understanding of her grandparents and their unreliability. That will actually be her gain. And very definitely, their loss.
Hug her, love her and make her realise that their behaviour is not reflection on her. It's not personal, directed at her. If anything, they would probably stir themselves more for her than for anyone else. Which isn't saying much, of course.
As I said - their failing. Their loss. Not Duckie's fault in any way, she needs to understand this. When it all boils down, the only person you can rely on for certain, is yourself. Anything you ever get from anyone else is a bonus to be grateful for and to not take for granted. And whenever other people let you down - that is their failing. You always have yourself and your self-reliance.
Marg