I think you might need to reread my explanation. No where did I discuss my reaction to my SIL's comments other than
telling my DD that I expected her when responding to an invitation to honor the attire request or turn down the invitation.
As another poster stated, this is an opportunity for teaching our children appropriate social interaction. I did not at any point state that I was asking opinions on how to respond to my SIL or what should I do about this situation. I was asking, as the poll states, how others respond to these types of invitations.
Trust me, no one here dislikes Halloween as much as I do. However, when I attend Halloween events, I do so in a costume, if the event requests them. I am in no way going to ask anyone not to attend or even have a discussion with my SIL regarding this. I just can't understand why a person that doesn't want to participate in an event such as this would even bother to come.

I turn down plenty of invitations to events that I think I will not enjoy. Likewise, I would not accept an invitation to attend the symphony with my Mother and show up wearing shorts and tennis shoes.