I don’t think you are in the wrong at all, and I agree that sharing the least amount of details about your plans with her the better. Obviously share information that she needs to know, but don’t give her anything that cause additional stress or over-reactions on her part.
I think I remember you posting that you were worried she would be upset that you got engaged so close together, but when you talked to her she was happy for you, is that correct? If so, hopefully when things calm down and after her wedding is over she will realize she was being a bit of a bridezilla.
PS – your brunch wedding sounds absolutely lovely!