Although I'm currently the only poster on this board, and the only member of this boards associated Renneker email list, I am going to go ahead with some of the results & questions of my own research, & hope to attract other people researching this surname (maybe even in related lines!).

T.D. Boaz, Jr.'s book on the descendants of GARRETT HENRY RENNEKER gives only 3 siblings for him: MARGARET, HERMAN JOHN, and CHARLES.

I didn't know about any other siblings until finding a enumerated with their father (surname given as "Renicker) in the 1850 census for District 99, Warren County, MIssouri. These additional siblings were RICHARD and ELIZA.

It may be possible that Eliza and Margaret are the same person, given that Margaret doesn't show up with the rest of the family in the censuses. But maybe she was the oldest child, and was already married by the time the 1850 census took place. And since the two youngest children (Herman John and Charles) are both stated on the 1850 census to have been born in Missouri, but other records indicate that Frederick (or Friedrich) emigrated from Oldenburg (Germany) with his wife and 4 children, all presumably born in Oldenburg, that also argues for Margaret and Eliza being separate people.

In later years, I've found separate census entries for Richard, Garrett, and Henry, as well as information that seems to confirm service of Charles Renneker (though he spelled his surname "Renniker") in a Missouri unit on the Union side during the Civil War. (However, Boaz's book states that Charles died at Gettysburg; my research indicates it's more likely he died at Vicksburg. I'll post on that later.)

But no more info about Margaret or Eliza, such as who they might have married, if they died young, what became of them.

-- Mel