Welcome to the board. We have something in common: I started to frequent these boards when my nephew was getting married, got fascinated by the sometimes-odd innovations of manners and style I was seeing, and got ‘hooked’ and stayed around to pontificate on the traditional understanding of etiquette. So be warned:-)! You may end up loosing all manner of time to this site! At least you will be a member already when your own children start planning weddings!
You are quite right, that only the persons named on the invitation are invited. Although it is always possible that the hostess herself didn’t know that and is assuming “come one, come all” prevails, I would go with the standard etiquette and not ask into it for fear of embarrassing the bride. You are also right that your adult child, and even possibly the seventeen-year-old, would properly get a separate invitation if invited. That is fine: a hostess has the sole and absolute right to invite whomever she pleases as long as she does not break up couples.
However, you also have sole and absolute authority over your own social calendar. Not everyone does enjoy “a weekend away from the kids”. Indeed, it speaks well of your humanity and your parenting that you actually enjoy the company of your children (and I sympathize: there are ten-year-olds and twenty-somethings in my extended family whose conversation I enjoy more than some of the thirty- and forty-somethings.) The old rule, that a wedding is so significant an event in the lives of both the couple and their community that acceptances are nearly mandatory, has largely fallen by the wayside as a consequence of the simple truth, that the ‘wedding’ of two people who have been living together in a conjugal relationship for years and enjoying social recognition of their couplehood just as long, is relatively INsignificant. Modern bride’s as a result face looser rules about who “must” be invited; which carries as a flip-side lowered obligation on invitees to accept those invitations.
If you want to decline, then decline. There is nothing rude or unsupportive about a polite refusal. Brides who choose not to invite children, or who choose to get married at a distance from their families, do so with the prior knowledge that some of their guests will not be able to attend under those conditions, and accept the fact. Even in the old days when weddings were actual rites of passage, inability to travel was an acceptable reason to decline.
Two of the saddest trends I have seen on wedding boards since I began haunting them (and the WeddingBee is better than most so I haunt it the most), are the general sense of entitlement some guests and some brides show, and the general disrespect to their older relatives. You are in a wonderful position to push back against that trend: let your mother know if you decide not to attend, and let her — whom I presume to be a competent adult who has negotiated the modern world for sixty or seventy years — decide how to manage her own commitment as a result. She can ask your brother for a ride — he is, after all, her son and she may have a better relationship with him than you do. I will not assume that “your brother would appreciate not having to take care of your mother while putting on a wedding” — taking care of a grown adult is not a chore and taking care of his own mother may well be — in fact I would hope to assume it would be — a pleasure for loving son.
When you decline if that is what you decide, give one of the old-fashioned acceptable reasons: a prior obligation, illness, or an inability to travel. Even if your “prior obligation” is a commitment to being there for your children to tuck them in at the end of every day, the bride does not need to know details that she might interpret as a judgement on her guest-list decisions. Make the simple polite excuse, and leave it at that. That is the first step in combatting the atmosphere of entitlement that many of these (usually novice) hostesses are navigating, with every friend and relative feeling entitled to voice an opinion on their choices. Then help your older children to see that they were not “entitled” to invitations and that close as they are to their cousin, she might be working with limitations that restrict her guestlist to an even closer small list of guests. Help them decide on gracious un-resentful ways of welcoming their cousin’s new husband into the family and recognizing her new status as wife and matron.