Melanie Wilkes is INFX, IMHO. It's stated fairly on in the book, the Wilkes and Hamiltons inhabit a bit of a dream world. I can't exactly say for Charles Hamilton....likey to be an NF too. Bear in mind I'm typing straight from the book.
Aunt Pittypat is no T either, what with her fainting spells, and dramatics.
I think the book version of Melanie is much more INFJ while the movie version is ISFJ (she is definitely no where near INFP though; she's all Fe). The movie makes Melanie look like an oblivious fool who consistently lets Scarlett take advantage of her compassion. Ni doms would see right through that, while Si doms would be a lot less willing to let go of their original impressions of someone, especially if the first impression is a good one.
The book, on the other hand, makes it seem like Melanie is much more aware of Scarlett's ill opinion of Melanie and her romantic interests in Ashley, and in general, Melanie understands Scarlett's perspective much more. In the book, Melanie seems to try to be the bigger person and maintain compassion towards anyone, despite Scarlett's ill opinion of her.
re: Aunt Pittypat:
Just because faint starts with the letter "F" doesn't mean you have to be an F to faint. That's a silly argument.
She gets worked up easily whenever attention is drawn to herself and something disrupts her typical daily life's structure. She is opposed to novelty and thrives on security and playing it safe (Si).
Thinking about it, I could see a case for ISFJ if you were to associate her constant anxiety over her reputation and maintaining others' proper perceptions of her with Fe. However, saying she faints and is thus not a T is a bit silly. Her fainting spells are consistently due to Si, which is a function of both T and F types.
I thought I saw Te in her with how structured and organized she keeps her affairs and how worked up she gets at the idea of her life's structure being interrupted and losing control of the situation, but I could entertain the idea of an ISFJ as well.