I wonder if we are perhaps connected.My great-grandfather was Karl/Carl Johan August Sandberg, b. 6 Jun 1866 in Karlstens Slotts, Goteborg och Bohus, Sweden.Karl emigrated to Boston with his siblings and his widowed mother sometime before 1895.Karl's father was named Frans Oskar Olof Sandberg and he was a Lutheran minister.I believe this Frans Oskar was born 26 Jan 1838 in Vastrelanda, Sweden.Rev. Frans Oskar married Anna Maria Olsson (b. 26 Jun 1844) in 1859.
I have found the birth record for Karl/Carl -- in fact that's how I discovered that his father was a minister.His father had signed the parish records for all the births that year.The rest of my information comes from Anna Maria Olsson Sandberg's Swedish Bible, which I own.
I don't find any indication of another brother named Franz Oscar.The Bible lists Ascel Konrad, Karl Johan August (b. 1869), Bertha Elvira (b. 1869), and Augusta Mathilda (b. 1872).But with the similarity of names and your family story of a Carl who went to Boston, I wonder if they might have at least been cousins?
I'll look forward to hearing what you think of all this.