I used “Sie” with my now mother- and father-in-law until the first time I visited them after we got engaged and my mother-in-law brought out a bottle of sparkli

I used “Sie” with my now mother- and father-in-law until the first time I visited them after we got engaged and my mother-in-law brought out a bottle of sparkling wine and asked if we could be on “du” terms. That was when I started using their first names. That was in 2008, and was, from what I gather, already rather old-fashioned. It was common enough in the past that there’s a verb for it that my mother-in-law used when making the offer: duzen.