Hi Patrizia,
Are the Christian names completely wrong? It used to be quite common here for babies to be given several Christian names - usually names of relatives - but not to be called by the first official Christian name.
I found that very strange when I came to Germany more than 30 years ago. As time went by, I found out that lots of people I knew were actually known by their second or even third Christian name. Several people I know use names - again, usually the same as a special relative - that aren't even on their birth certs. Rather weird.
I was talking once to a friend name Ede. I asked him what it was short for, and he said he didn't know. I said I found it weird that he didn't know about his own name, to which he replied that it wasn't his name - his name was actually Matthias. I asked why he used Ede and he said his family had always called him that - it was his dead grandad's name.
So it could be that your ex in-laws were known to their friends and family by names which may not have been the (first) ones as shown on their birth certs.
It might be worth contacting the local offices in Sinzig:
The responsible office appears to be the Ordnungsamt, in the Rathaus (town hall) in Sinzig:
ordnungsamt@sinzig.deThey are responsible for BMD certs and, according to their website, Ahnenforschung (genealogy).
https://client.rlpdirekt.de/public/svsinzig_bis/home/abteilungen-von-a-z/I hope this helps.
Best regards,
Karen