If I remember my etiquette from my wedding 8 years ago, children beyond a certain age still living in their parents house (can't remember the age, but it might be 18

) should receive their own invitation. Adults living on their own, married or not, should certainly receive their own invitation. If children under the age where they get their own invitation are invited to the wedding their names should be listed on the inner evelope of the invitation (i.e. Mr & Mrs Smith (next line) Miss Jane Smith; Master Joseph Smith) If the name are not on the invitation they are NOT INVITED. I don't care how old you are.
I know you did not ask this, but....2 single adults living as roommates should also receive their own invitations to events such as this. And, if a single person is invited to bring a date it is proper form to get the name of the person first and address the invitation to both of them (none of this "and Guest" stuff).
If I were in this situation I would assume I was not invited to the engagement party, unless it was one of my tackier relatives and the I would assume they didn't know any better and go anyway!

If I got the explaination given here I would go to the wedding with no hard feelings. Again, everyone doesn't read Miss Manners for fun.