How many other weddings has the younger sister been to? I suspect that she just wants to go to A wedding — whose it is doesn’t bother her. If you haven’t been to many/any weddings they seem very exciting.
I’m surprised no one else has picked up on this point - especially since OL herself says of her elder daughter:
Didn't the bloody invitation arrive when I was out and it was opened in front of her. She loves weddings and is devastated.
Note that it’s “She loves weddings and is devastated”. Not “She loves Cousin Charlie and is devastated” or “She thought she was close to that side of the family and is devastated”. Missing the wedding itself seems to be the upsetting part.
It makes me wonder, just how traumatised is the 15 year-old about a barely known older step-cousin not considering her family? Is it in fact the OP who is really the one most upset by this, and is taking this out on her younger daughter under the guise of worrying her older daughter will be upset?
I know my previous posts will make me seem very much Team 12 Year-old, but I do understand why OP is upset, and I know I personally wouldn’t have taken this approach at my wedding. But I still feel that, in her efforts not to upset her older daughter (which may actually be more to do with her own feelings on the subject), she’s missed how much she’s upsetting her youngest daughter.
She could have said, “I know you’re disappointed not to be going, but I really feel strongly that we should have been invited as a complete family unit. I hope you understand why I feel like we should all say we’re not going”. Instead, OP is actively angry with her daughter for not falling into line on this. She won’t even acknowledge her disappointment - she’s actually angry with her for feeling it. How does this not give the message that the older daughter’s feelings are the more important ones?
The OP’s husband’s attitude isn’t helping either. Rather than being angry with the bride and groom, he’s blaming his brother; turning on the amateur dramatics, saying he will “never forgive him” for a decision he didn’t even make. Has he considered whether this is really worth it when his stepdaughter’s disappointment may actually just be about missing a party?
It seems to me that making a stand - and a big dramatic point - about this has actually become more important than anyone’s actual feelings. And the 12 year-old daughter comes a very clear bottom of the list.