I think a lot of you may be misunderstanding “Miss Manners.”
Yes, it would seem that the OP was in the wrong at first glance. But The rules are a bit different in a case where a “guest” is damaging the host’s property or the other guests.
In this case, Dorko was, in fact, damaging the host’s property by bringing a stinky cigar into a non-smoking house. And he was damaging the other guests by exposing them to stinky smoke. Yes, I know that a lot of you are thinking “aw, a little cigar smoke won’t kill a kid,” and that may be true, and it is the parents’ choice on whether to allow their kid to be exposed, but I think Maureen felt that the parents’ of the kids (Janet and Mrs. Dorko) were reluctant to stick up for the welfare of the kids for fear of a confrontation with Dorko. So, she stuck up for them.
Furthermore, the sister of the hostess, especially if the relationship is close, has a slightly different role than that of an ordinary guest. Presumably, Maureen was helping put on the party, especially since one of the guests of honor was her mother. Therefore, she is in the role of semi-hostess.
Given these considerations, the standard rules of host/guest are bent a bit.
Now Miss Manners herself would probably say that Maureen should have gotten Dorko to put out his cigar with an icy stare and a cool remark, but it’s easy for her to come up with that stuff when she’s in front of her typewriter. Who knows what she would do if confronted with Dorko himself!
So, good on you, Maureen! I’m glad you told that idiot off.
And I agree with Queen Tonya–Why did they invite someone that they knew would act like that? Because Dorko may have been offended? Dan may be a good guy, but he needs to learn to put the welfare of his friends and family above that of his softball team.