Some people refer to them as the « in-laws » but I can’t think of anything shorter than that! Hope this helps ☺️

As a joke you can say 'the outlaws'!

Nah, since they are a bit formal people don't usually shorten them.

Not really and nothing that would be used in anywhere formal. In the right context online, like a parent group, MIL or FIL might be used but the context would be essential as it is not an often used abbreviation.

Hi Dinghui, I believe the most common acronyms are MIL for mother-in-law and FIL for father-in-law.