In my opinion, etiquette says al ot of things, but never offers exactly the advice you need. Such as “don’t invite someonver over 18 on their parents invites” however, if they all live in the same house, I don’t really see why it matters (that’s just me though). I’ve ALWAYS been included, and still am, on invitations my parents get, and I’m 33.
And with SO’s, I hate it when someone says “well, they MAY be in a relationship at the time, you never know. Well, if they aren’t one when the guest list is planned, then that’s that. Unless someone has a whirlwhind romance in a couple months, I really doubt anyone will meet, date, movie in with, AND marry someone in a couple months (yes, I know its happened, but hey, this is just my opinion).
And its family. To me, it depends on the family. If they’re super casual, then whatever goes. If they’re the more formal type, then that goes, I guess.
When my best friend got married, she sent invitations to “parents” (such as aunts, uncles, etc) and included “and family” regardless if they were over 18, lived with their parents, or what. but it was a simple wedding, 99% family (I think I was one of 3 friends invited) and no one cared.
Let me also add, that around here, it’s just different because it’s nothing for people to keep living at home forever. I just don’t see the reason behind if a couple has 6 grown children living at home to send 7 invitations to the same house. But again, that is just my opinion and I’m not imposing it on anyone.