Was this person rude to behave like this or is it perfectly fine social manners, read below?

So my great aunt was invited to a family cook out my grandmother was having. The invite was to my great-aunt. She brought her son who is single and has no kids. My grandfather, her brother, said there was nothing wrong with her bringing him and the party was casual. My Mom and my grandmother were mad since there... show more So my great aunt was invited to a family cook out my grandmother was having. The invite was to my great-aunt. She brought her son who is single and has no kids. My grandfather, her brother, said there was nothing wrong with her bringing him and the party was casual. My Mom and my grandmother were mad since there was not a formal invitation to the son though they both said it was nothing against the son personally.

So was the great-aunt rude to do this since the party was casual? She did not ask my grandfather but it was a casual party and she occasionally brings him when it is something casual. He is 37 and never married, kind of keeps to himself. A nice guy and a little chatty but not loud or anything.

I ask cause my Dad sometimes asks me to join him at casual stuff my two aunts have. One I know does not mind but now I am afraid the other might. Was my great-aunt rude today?

Of note---When it was a formal country club dinner for my grandfather's 85th she did not bring this son along and just came solo (as that event was more formal).
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