This comes from the following excerpt: “But are his needs any more shocking than the needs of any other animals and men? Are his deeds more outrageous than the deeds of the parent who drained the spirit from his child? The vampire may foster quickened heartbeats and levitated hair. But is he worse than the parent who gave to society a neurotic child who became a politician? Is he worse than the manufacturer who set up belated foundations with the money he made by handing bombs and guns to suicidal nationalists? Is he worse than the distiller who gave bastardized grain juice to stultify further the brains of those who, sober, were incapable of progressive thought? (Nay, I apologize for this calumny; I nip the brew that feeds me.) Is he worse, then, than the publisher who filled ubiquitous racks with lust and death wishes? Really, no, search your soul, lovie--is the vampire so bad?” ― Richard Matheson, I Am Legend and Other Stories The author means that he has just said something negative about those who provide alcohol to others knowing that it is harmful to the brain, even though he himself drinks a lot of alcohol to numb his own brain and thoughts. So, yes, this is a creative literary parallel to the idiom "to bite the hand that feeds you." The author (or character) is simply applying the idea specifically to alcoholics and brewers rather than to dogs and masters.

It sounds like a play on the much more common expression "don't bite the hand that feeds you". Which means don't be mean to someone that is important to you or, if you are a dog, don't bite your master's hand if you want to get fed again and so on. "Nip the brew that feeds me" indeed sounds poetical. "Brew" is beer and a "nip" is little bite. So maybe this is about someone who needs alcohol as sustenance, but knows it has to stop? I am guessing here obviously, but maybe with knowledge of the context you are able to sort this out.

Hey! the explanation comes under a subjective literary analysis. Nip means biting, and the phrase leaves to the interpretation of the image of nipping a brew that is feeding you. As I have no context I may not be able to give you an insightful interpretation of the fragment, but I think is just a matter of interpretation. Hope it helps!