Addressing wedding invitations to engaged couple

Niece getting married. Her uncle is engaged to a woman who does not yet live with him, of course. Uncle has one child (over 21) living at home, single, and another living on her own (over 21).

Question #1: How should the invitation to the engaged couple be addressed? To both of them at the uncle’s address or one invitation to each of them at their separate addresses?

Question #2: Should the uncle’s invitation include his two children and be sent to his address and a separate invitation to his finance at her address if the engaged couple are to receive separate invitations?

Question #3: Is it customary or obligatory to invite a guest for each of the cousins even though they are not presently going with anyone seriously?

And Question #4: If one cousin has a serious boyfriend who she has dated for 6 years and the family knows they are planning on getting married but not formally engaged yet, is it wrong to invite her longstanding boyfriend if none of the other cousins are being allowed to bring casual boy/girlfriends?

Rebecca Black, Etiquette By Rebecca

Dear Addressing,

#1. The invitation can be addressed to both of them at his home address. Because she is his guest and doesn’t live with him, it would be best for her name to be listed under his.

#2. Separate invitations should be sent to each child over the age of 18.

#3. If the cousins are not seeing anyone, an ‘and guest’ is not necessary. Many will allow close family this option, but it isn’t obligatory.

#4. It is customary to invite significant others whether or not they are formally engaged. So, if any of the cousins are in a relationship (a known relationship) these cousins should be allowed to bring this person as long as there is room for them.

Best wishes,