Have I fallen into "fitna" or "ghiba" by inquiring about my uncle's wives and conversing with them, then publishing that in a story on "Facebook," followed by a change in interaction thereafter?

Your question about writing a novel that includes information about your grandmother and your uncles' wives involves several issues:

Firstly: If the person who informed you does not approve of what she said being published, and if publishing it would harm her, then it is not permissible to publish it.

Secondly: If the كلام (speech/information) is vague and the individuals mentioned cannot be identified, then it is not considered prohibited backbiting (gheeba). Otherwise, it is backbiting.

In general, whether what you did was backbiting or sowing discord (fitna), you are advised to seek forgiveness (istighfar) and repent (tawba). Repentance is the first, middle, and last station for the believer, and through it, success (falah) is achieved. Moreover, whoever does not repent is an oppressor (zalim).

There is no specific expiation (kaffara) for backbiting, but good deeds in general atone for evil deeds. If you have wronged someone through backbiting or divulging a secret, it is prescribed for you to seek their forgiveness.