Ah, the famous scene on the porch in Gone With The Wind.
Scarlett's grammar is quite correct. Until the Tarleton twins mentioned the barbecue that they were all going to the next day, Scarlett had not given any thought as to whom she would actually be sitting with at the barbecue.
To put this in other words: I hadn't thought about whom I would sit with before this moment. Now that you ask me, I haven't decided yet (because I hadn't even thought about the barbecue till now. I'll think about that tomorrow."
The first verb is past tense: in the past, right up to the moment at which the barbecue is mentioned, she had not given it any thought.
The second verb indicates that the act of deciding is now an ongoing action: I have not decided as yet, but I will think about it tomorrow and make my decision then.
Interested in Language