My Grandma Doesn’t Want My Stepson Coming to the Family Gathering

I (F28) am married to my husband (M30) who has a son from his previous relationship. Stepson is 8 years old (SS8) and I’ve been in his life since he was 11 months old.
My SS has only been to my grandparents house once, when he was very little. And from what I recall it was a calm uneventful afternoon. When COVID happened, my grandparents aired on the side of caution, which resulted in excluding SS (due to the petri dish of germs at daycare – completely understandable). However over the years my grandma found new and different reasons to exclude SS from family gatherings. With this most recent family gathering being planned she said, “SS cannot come since there is too many breakables in the house.” Being around SS a lot more, my brother (M29) and my dad (M57) all vouched for SS, saying that “SS is very well behaved and will not be a problem”. My brother said he would even make it his personal duty to run around and play with SS outside if he had energy to burn off. But my grandma still said no.
Although my grandma’s decision upset me, I did not want to push the boundaries she has set in her own household. Even though I know SS won’t be a problem, I don’t want to piss my grandparents off and disrespect them, especially in their late life. My husband stated he will no longer be attending if SS cannot come, which is reasonable. However I decided I’ll still attend without them because I don’t see my grandparents often and I don’t know how many more opportunities I’ll have left. My husband wishes I’d stay home, but also understands.
Well my dad could not drop it. He stewed on it for several days, and I ended up sending a message to my grandma that SS should be included because he’s my family, and well behaved. He claimed my grandma was treating me as a second class citizen and disrespecting my family. My grandma got pissed and canceled the whole family get together after that.
I feel touched that my dad and brother would stick up for my small family unit, but I have mixed feelings about pushing the boundaries people set in their own homes. I just wanted to keep the peace and everyone feels bitter. (And of course we don’t tell SS any of this)
TLDR; Grandma doesn’t want stepson coming to the family get together. I respect her boundaries since she’s hosting. My dad called her out on excluding my family. My grandma then got pissed and canceled the whole get together.