Seth never demanded of Jane. He wanted a brandy and cigar but Jane said no.
He never said not to drink any type of alcohol.
Seth said smoking is a "dirty habit", and so it is, and that she would be healthier without it. When she died her lungs were in good condition because of her beliefs, though. Seth said her beliefs about being a writer--the romantic smoking writer, pushing forward with work through cold coffee and cigarettes and a host of other personal reasons, no doubt--were too strongly connected for her to overcome.