YADNBU. The DM is being selfish to deny her DDs the experience,just because she won't be there. They aren't babies, they are young women, I was shopping in Paris with my aunt at 14 & frequently travelling to various places on my own. My DM was working away from home at close to that age too
Parenting for some has gone nuts, so smothering & trying to keep adult kids as babies, I really feel for those kids & the parents too, who won't know their kids as teens if they keep treating them like babies, because they grow up whether you like it or not & if you can't roll with that & let them grow, they will still love you, but they won't be liking you very much or wanting to spend time with you when older teens
To those with the worries of kids going abroad without a parent. Give your head a wobble, it's perfectly easy & safe to do. These girls are clearly not babies
I took a friends 7 yo to Paris Disney for DDs birthday, we just needed a bit of extra paperwork, no big deal. The only issue we had is that the DD clearly wasn't ours as she's a completely different skin colour, so we were checked & questioned a lot more thoroughly, which I had no problem with when they explained it was to do with child trafficking, refugee kids etc.
I agree that your DH is being very weak & should speak up for his DDs rather than roll over & agree to anything the DM does for the sake of keeping peace, how does he even know It's going to cause a row, unless the DM has form for kicking off, withholding contact, which isn't something she can do at 14/18 anyway & doesn't sound that way from your posts.
If it were me, I'd be tearing DH a new one, given that you are clearly very hands on with your DSC. I'd be very upset that I was good enough to take up the parenting slack from him, but not to be trusted on a mini break. That would make me very cross & he'd know about it.
Not sure what you can do, but I don't blame you for being upset