I agree with PP - there is probably a broad gray area in all of this...
I can see MOB saying "no don't come we don't have a seat for you" and Conor texts the bride who says, "oh pooh, come on Conor we will make room". Then he shows up with Taylor.
Bride is thrilled, MOB not so much. So Conor and Taylor leave during dinner and come back when Bride texts, "hey dinner is over, come on back we are starting the dancing". Bride wants her cousin at her wedding (and possibly celeb), MOB wants wedding to be all about the daughter, not about celeb dating inconsiderate nephew.
Then to make sure everyone knows that MOB is ticked, she reports the bad etiquette to newspaper. Publicist for celeb denies it because as far as Taylor is concerned - she was invited by the BRIDE, what is publicist supposed to say?? Then KLG blurts it out on her morning news program - she is asked if she went to a wedding and in great detail tells what her step daughter tells her happened - she doesn't ask the bride, Conor or Taylor but reports what she "thinks" happened.
And I'm with everyone else where was Conor's father and/or siblings in all of this? Surely, someone thought to ask a teenager what his plans were or if he had responded, etc.? And of course the bride thanked them for attending - what was she supposed to do...scold them for showing up???
Pretty much everyone in the tale had an etiquette breach...