Wicked Stepmother by Michael McDowell

Trade paperback format.  Co-written by Michael McDowell & Dennis Schuetz, originally published under the pseudonym of Axel Young.

Seducing a proper Bostonian was easy. Making him die of a heart attack a week after the wedding was hardly any trouble at all. Now she has money, social status, everything she has cunningly schemed to get since she was a poor little girl. Everything except the Brookline mansion and the multi-million dollar trust fund left to the three children. 

But what wicked stepmother couldn't get rid of three children? 

Jonathan, too smart for his own good and suspicious about his father's death, has to be the first to go. Then there's pretty, spoiled Verity, hooked on cocaine and perpetually drunk - who'd ask any questions if she died suddenly? And clever Cassandra, just out of college and beginning a career as a literary editor, will simply have to get over being dead. 

Wicked Stepmother (1983) is the second of the camp classics written by Michael McDowell (co-author of Beetlejuice and The Nightmare Before Christmas) in collaboration with Dennis Schuetz. Five of McDowell's classic horror novels are also available from Valancourt.