In-laws not accepting my child

I know this is kind of off topic for the October bumps but I'm desperate for advice. My soon to be in-laws invited us out to the lake for a couple days. The catch is that I can go, my fiance can go, and our 22 month old daughter can go. But my 7 year old can't. They wouldn't say why but they kept making stupid excuses. She is so hurt and it has me furious.