It may be that a Gelis brother and sister married a Kagels sister and brother.

I have found the 22 May 1876 marriage at Chicago's St. Matthew's German Evangelical Lutheran Church of William Gelis (the brother of Henrietta Gelis) to Johana Kagels (was she the sister of the Charles Kagels, who married Henrietta Gelis?).

So I am definitely interested to hear from anyone researching either of these lines.