Overindulgent grandparents?

My parents really do dote on my toddler. But sometimes they can go to extremes. Just the other day, we were at my uncle's place and they handed him an ornament of my uncle's (without permission from him and which I know is priceless to him) to play just because my toddler was wanting it. Then, subsequently, they let him bang on my uncle's piano again without permission. I put a stop to it and of course my baby started yelling, to which my mom's response was to scoop him up and console him, saying all the while " mommy". I was really quite mad at them. My boy spends the day with my mom while I am at work and I'm worried about discipline with him. I've tried talking to my parents but they think I'm overreacting. Any ideas what I can do?