Unless I got mixed up, one of your later posts said you "don't know" if other children who aren't in the bridal party are invited.
If that's correct, then I think you are being very self-centered in how you're looking at this. You don't care about their overall criteria for who is invited and who's not, or if they actually even ARE slighting your daughter at all or not. You simply want your own way in the middle of your husband's daughter's wedding and only care about that. Well... shouldn't you really be a very MINOR player in that, rather than someone eager to throw down the gauntlet and damage your husband's relationship with his daughter?
It is VERY pertinent to know if other children are invited or not (besides the ones who are actually in the wedding) because if they are not, then you being offended equals you being overly self-absorbed. Your husband's daughter isn't very close to your daughter and bridal couples have a lot to consider, besides only how the criteria they set for the guest list lines up in each relative and close friend's current nuclear family.
If it really needs to be explained, couples have to limit the guest list, due to money and space constraints. When they go by "rules" like "no children- except, of course, the ones who are in the wedding" then people might not like it but it would be unreasonable to not think it's FAIR- because it is. When the couple starts making exceptions is when they get into trouble with many others on the guest list, and understandably so.
So before getting into this any further, I'd stop right there and FIND OUT if other children are being invited or not. If not, then don't cause trouble. Just quietly excuse yourself from the wedding and spend the day doing something fun with your older daughter. Don't cause trouble unless you want trouble.
Also, on reading your take on your husband's position, my feeling was that's only "your take" on it. He may be unable to stand up to anyone or he may not care that much or he may think you're causing unnecessary problems.
And, again, there have been a lot of posts so I could have missed something, but I did not see anything about how your DAUGHTER feels about this, only how YOU feel about it. She's fourteen, she may see it completely differently than you do and not care about it at all or even be glad she doesn't have to bother with going to a boring wedding.
Another possible issue that is very real for the couple to deal with but do not revolve around YOU are the wishes of the bride's mother, who does and should come before you in the wedding of HER daughter.
Yet another one is to remember that you are dealing with a young adult who is inexperienced in planning weddings. As the older and supposedly wiser adult with more life experience, I think it would go a long way for future relations for you to consider being gracious and simply stepping aside on this one instead of making it about you in any way or making trouble when weddings are already so very stressful for a young couple to navigate.
So. Before proceeding with anything else on this, I suggest taking a DEEP BREATH, then stepping back and re-evaluating the overall situation instead of only thinking about yourself and your family and what you want. Good luck.