Do you know the full maiden name of Violet's mother? Was PETERS her maiden name, or was Violet's grandmother Barbara married more than once? Where did Barbara Peters live?
Do you already have census information for any of the people mentioned in your post? In 1910, a William HOLZAPFEL with wife Adelina and son John are enumerated in Chicago.
John HOLZAPFEL (born Aug 1 1894) registered for the WWI draft (1917); do you have his draft card scan? William HOLZAPFEL (born Dec [11?] 1972] also registered for the WWI draft and lists Adelina as his wife.
Have you procured death certificates or obituaries or probate/will documents for any of the people you list? One or more of these might mention Violet