A boy finds out his sister who ran away is now a "Stock". Possibly published in early 1980's or late 1970's
I'm pretty certain that the book in question is "Through Darkest America" (1986) by Neil Barrett.
Read this one many a year ago and have been looking for the sequel. "Dawn's Uncertain Light". "Stock" isn't like people, because stock doesn't have souls. At least that's what they told themselves.
As described by The SF Site, the novel's setting matches your description:
Over one hundred and fifty years ago there was the Great War, which wiped out most of humanity, brought down all the old cities and technology, and eliminated just about all animal life. But humanity has struggled its way back to reclaim the land. In Middle America, life continues, farmers grow their crops and raise their stock, and try to make an honest living in the world. ... During the War, most animal life was wiped out, and over time it's come to be believed that this was God's way of cleansing the "unclean flesh" the previous, corrupt, generation of men had eaten. Now all meat comes from stock, genetically bred humans with no intelligence or speech.
and the protagonist discovers that his sister has become one of these:
In this harsh land, Howie undergoes brutal punishment and is literally beaten into adulthood. Through it all, the only saving grace is the knowledge that his younger sister Carolee has gone to Silver Island, the idyllic paradise the government has created to help restart society. But of course, this is not a world of happy endings and Howie makes an awful discovery about the true nature of Silver Island and its connection to the stock that people eat on a daily basis.