Stressed About Visiting Family

My husband and I are planning a trip to visit family. Last visit, we were both really upset with how they acted around our baby (now 16 months). They compete for his attention--saying his name loudly, over and over. They are aggressive in their efforts to hold him, as if the quality of their relationship with him depends on it. During the last visit, at one point, he reached out to hold his great-grandmother’s hand. She took this as a license to pick him up, but when she did, he began crying, arched his back and reached for me. I went to pick him up, and she pulled him away, telling me she wanted to calm him. It was clear that he didn’t want to be in her lap, but I had to ask her twice to hand him over before she let go. I have told them repeatedly that we pay attention to his signals, that it’s important to us that they only do what he is comfortable with, but it’s as if we’re speaking different languages. We want to be proactive and talk to them before our upcoming trip. What do we say?