Grandchild playing favorites

Backstory
I am very close to my oldest. I was his caregiver from 4 weeks until 8 months. He had a rough relationship with his mother due to postpartum depression. I became very protective of him because of this. He still has a strained relationship with his mother as their bond is not as close as it is with his sibling. He will often prefer to be with me than anyone else. His paternal grandparents are very much active in his life and I have seen her get her feelings hurt or be jealous of our relationship. She will make comments like “what is so special about Mimi”? And act hurt. I think it makes him uncomfortable, although he hasn’t voiced this to me. It’s also become a tit for tat. If I make plans to go spend time with them, they will do the same the following weekend. If I buy them something, they will “one up” us with something. Which I guess is fine, just annoying. We live 1 1/2 away from them and I will make plans to get them to stay a few days and I am expected to make room in my plans for them to be taken to the other grandparents for part of that stay. Although they never do that for me. I often won’t know they are In town til after the fact (which is fine, again just annoying).
Well I made plans and went and picked them up for the week for summer vacay. Again, I have to take them to their other grandparents so they can spend a couple of days with them before I take them home. (They never go get them themselves). It’s always during my planned visit.
Well my grandson came down with stomach flu yesterday and so my grandson messaged his grandmother letting her know he was sick and wasn’t sure if he was coming to her house. During this conversation he fell asleep. She wrote back asking if he was still running a fever ( she’s trying to figure out if she needs to ask for the day off or if he’s going home). So I answered back for him while he slept.
I saw on his previous message to her during a different convo ( I know I shouldn’t have read it, but it was right there on his messenger conversation) he told her he thought she was the best and that she was the highest on his love list and that I am the lowest. And please don’t tell me that because it was mean for him to say that. Her reply was”I love you more”.
Question: how do I handle this? Do I say anything or just move past it?
Obviously my feelings are hurt. I feel like I deserved it for reading it. But I really didn’t think much of responding to her message and I feel like I always go out of my way to help her with her relationship with him because I knew her feeling had been hurt in the past. She didn’t even say anything to him about how that kind of talk can be hurtful.
Sorry for the novel. If you read this far, thank you.