For example, if you were married to someone with children and had your first child with them also. Would you expect your parents to treat your step children exactly the same as they do their biological grandchild?
Not necessarily talking about being nice/kind/talking to them when visiting as that's obviously standard but things like sleepovers, days out, taking them on holidays, present buying on birthdays or Christmas, being more interested or asking more, say in their achievements etc...?
If the step children had two involved parents and sets of grandparents on both dad's and mum's side already.
YABU - all should be treated the same.
YANBU - it's expected that grandparents will favour their grandchildren in some ways.