invitation addressing confusion!

i'm so confused when it comes to invitation addressing. ettiquite and when it comes to kids over 18 living at home. For instance, my cousin is 21 living at home, but he isn't dating anyone and i would not want to invite him with a random date. Is it okay if i can just address the invitation to:

Outside:
Mr. & Mrs. John Smith (uncle and aunt)

Inside:
Mr. & Mrs. Smith
Joseph Smith (cousin)

And is the inside supposed to be less formal? like:
Uncle John and Aunt Mary

I've seen it so many different ways when i googled it so i figured i'd try to pick your brain! TIA

Hmm I don't know but I didn't want to view and not respond

I don't know what etiquette calls for but personally I would put the same on the outside and inside:

Mr. & Mrs. John Smith (uncle and aunt)
Mr. Joseph Smith (cousin)

Like I said, I don't what etiquette calls for - JMO.

i did the inside envelopes less formal because i didn't feel comfortable saying Diane & Kenneth for my Aunt Diane & Uncle Kenny.

for a cousin who is 21 & living at home i would send them their own invite, even if they are not invited with a guest.

I would put the Smith Family on the outside.

THen on the the inner envelope list them

Aunt Mary, Uncle John & John jr.

I have older cousins too who are sometimes at home and other times at college. BC we will be sending my invites when they could be in transit or moving my parents told me to just send one to the family..