Question: What massacre was Peter Gusenberg's brother a victim of?
Sentence (from "Peter Gusenberg"): Peter Gusenberg a.k.a. ""Goosey"" (September 22, 1888 – February 14, 1929) and his brother Frank were German-American contract killers and members of Chicago's North Side Gang, the main rival to the Chicago Outfit.
Relevant evidence: Yes