The Catholic Church records for Wurmlingen are online at Matricula. There is a family register for the family of Johannes Merk at this link.
https://data.matricula-online.eu/en/deutschland/rottenburg-stuttgart/wurmlingen-st-briccius-dekanat-rottenburg/M+0275%252C+B+014/?pg=120Child number 3 is Katharina, born 7 August 1764, died 6 April 1832. There is a footnote by her name. At the bottom of the page the note states that she had an illegitimate son on 9 November 1795, born of N.N. [Unknown] Portei [Franzes = Frenchman?]. It also states that Nikolaus had his first communion in 1827.
The number at the end of the entry is the family page for Nikolaus (p. 349).
https://data.matricula-online.eu/en/deutschland/rottenburg-stuttgart/wurmlingen-st-briccius-dekanat-rottenburg/M+0275%252C+B+014/?pg=179This entry indicates that his father was N.[Unknown] Portei ein Grazoss[?]. I'm not sure of the spelling of the last word or its meaning. Under Nikolaus' name is the note "Ausgewandert nach Nordamerika im Jahr 1832". He emigrated in 1832. No children of this couple are listed.
I hope this helps. At least you have a family group for Katharina Merk.