This was a misread of ettiquette on the mother of the grooms part. There was a time when unwed women would be invited under their parents invitation-however as you are living on your own it was improper of her not to send you an invite.
Me thinks they may have run short on invites and were trying to cut corners to stretch out to as many people as possible. When you think about it this was 4 invites for one family. Although improper if this is what she wanted to do she should have placed your name on your parents invite so they would know they had the option to rsvp for 3 (mr & mrs jones, sibling # 3).
My question is why didn't any one ask before the event? It would have easily been resolved then (this isn't the same as calling and asking if your new boyfriend that no one has even heard of can come or insisting that your 2 year old be included in a formal black tie affair)- in this case if everyone in the family was invited it would seem odd that one sibling would be left out & a quick phone call would have resolved everything.
That said, it appears it was not an intentional oversight so yes I would go the wedding and chalk it up as an honest mistake.
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As for the original poster:
1) I too know people who married in early 20's and are still acting like honeymooners - its all about the individuals and not age.
2) While marriage is between a man and women it does incorporate a "bonding" of the families - if your fiance wants to make it "official" by asking your dad or making your family a part of the proposal let him. It doesn't hurt a thing and will go a LONNNNG way with them. No matter how long you've been on your own or what your relationship is with your dad on some level you are still his little girl and this will go a long way to help him start the process of letting you go (unless you are dealing with a dysfunctional situation that entails maintaining your distance).
3) Your ring is your business-just be prepared for the haters who will comment on your ring not being a diamond -take deep breaths and don't slap them - you have to keep your nails nice for your wedding day pictures
