We have no hard and fast rule, often depends on the occasion and who made the plans / invitation.
Generally with my Dad & wife, they pay, they like to treat us and often invite us to places we wouldn’t generally extend our budget to. In return we try and pay for things like breakfast out, a round of drinks etc but this generally requires us to rush in as they still like to offer.
My mum & husband, varies, we’ve treated them, then us and we have spilt the bill.
DHs parents rarely eat out, if we go to a cafe / fish & chips at the seaside, we usually end up paying, not sure why as they are fair better off then us!
We pay for the adult kids if we invite them out, including any partners / friends if we ask them too. My mentioned this recently as he ends up paying for himself & usually his gf if they go out with her parents where as we always pay.