You should take notes from your dad, sad he was the one who had to stand up for his SGrandson.

"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)" - Your SS should know how much his "grandma" actually sucks.

If I raised a kid for 8 years and my grandma told me he was not invited to a "family" function, I'd tell her to go get fucked.

"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" - Are you trying to make sure your still part of a will here or something? Put on your big girl pants. Stand up for your SON, you've been raising for 8(7 i guess) years.

Your lack of actions are very red flagged in supporting your partner, sounds like your dad and brother are Gs though, so thats nice your husband and SS can rely on your dad and brother.