Not sure how to handle this one: What if you know and want to invite half of a blended family because you actually do not know the newly added half? For instance, you know the wife/mom all of your life and her 2 adult children (18 & 20 yrs old). I am assuming you would invite her new husband, but would you have to invite his 2 daughters, as well (19 & 20 yrs old)?