Spellings for this name found in Milwaukee's German papers include: Robrahn, Robran, even Robraan, and I've indexed matches to this posting for Chas. Robrahn family. I've referenced other surnames with Robrahn: Foesch Pape Küsel Kuesel Techel.
Indexing Milwaukee's German newspapers continuously since 2007 ...
Gary Rebholz, compiler/editor
"Milwaukee's German Newspapers; an index of death notices and related items" (1844-1950).