Martha Hasel Wise

Murdered at least three members of her immediate family, including 
her mother, for opposing her marriage.  Medina, Ohio.

Martha Wise of Ohio committed her crimes at the age of forty. A
spinster when she found her true love, she was overjoyed. But, her
family objected to her choice, as did her minister. (Her beau, Walter
Johns, was a much younger man.) In revenge, she fed three of the more
vocal family members a deadly dose of cyanide and torched the church.

http://www.crimelibrary.com/serial7/widow/2.htm

http://www.familyofmen.com/female_serial_killers.htm

http://www.mail-archive.com/law-issues@mylist.net/msg01227.html