Both of my parents got married when I was an adult (actually very recently), so I technically have step-siblings, but I don't really think of them as family. My parent's spouses are family who I am close with, but their kids are pretty much strangers to me. We've met a handful of times (both mom and dad's side). They never even crossed my mind when I started doing the list, but I just thought of it now. Do I have to invite them?
edit: I'll add that they are all also adults, a little bit older than me, and two out of three have kids of their own. One has a bad relationship with my mom, the others get along with everyone fine. I have nothing against any of them, but no burning desire to be friends. One lives 8 hours away, and the others are about an hour. I haven't asked my parents yet because I wanted to hear some outside input first.