How should I address my in-laws?
I'm interested in the case when the person is about the same age (or younger), but it's someone you barely know.
Should I use Sie or du?
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By social convention, it’s the job of the older and usually “higher ranked” individual to determine the initial style of address and also changes to it, which will almost always make it less formal. In this case, try to wait until one of your parents in law addresses you with either du or your given name – or, even better, let your spouse clear it up for everybody at first meet! When they want to address both you and your spouse, they’ll have to choose ihr (not Sie) either way, because not dutzing your children died out around WW1. Children in law are usually treated the same as children, but it’s sometimes more complicated and awkward when meeting them for the first time which is usually long before marriage.
In most cases nowadays, outside very stiff circles, everyone will address anyone with du at family celebrations like weddings, birthdays and funerals. When introducing yourself to someone there, use your full name anyhow unless the other one already did it differently, then follow suit.
As a general observation, German habits tend to be more formal (Sie, Frau/Herr Family Name) in a professional setting, esp. workplace, than in English-speaking areas, the US in particular, and they’re less formal in private, personal relationships.