ETA: Reading your "So What Happened," let me clarify that you don't need to do everything I listed if it's too extreme! Just pick some good ones.
Also, I have no idea why you're getting some pushback because your 18 year old is still at home. My son turned 18 during the summer between junior and senior year, and was almost 19 when he graduated. No big mystery - he simply didn't start kindergarten until age 6 because he was a 3-hour afternoon napper until age 4.5, so he wasn't ready to be in school during those hours at age 5. So there are a zillion simple and good reasons why an 18 year old can be at home and in high school.
Original answer: I get that this is miserable. But the good news is, it’s easily resolvable and you have all the power!
Stop being the referee. They are 16 and 18, and you have pretty much raised them to the age of maturity. They can work it out themselves, and not involve you.
You have done some enabling here: you’ve allowed locked rooms because they haven’t developed any respect, and my guess is that they still have access to phones, internet, meal privileges, car privileges (older one) and ferrying/ride privileges (younger one). You’ve also paid activity fees for school sports/clubs, bought them the coats and hair accessories and chargers they are fighting over and stealing, and listening to their BS. You’ve allowed the culture of “it’s not fair because of what happened in 2014…”
You have 2 choices:
Option 1: Give them 24 hours notice that you will be confiscating all of the contested items and anything else in a similar category. So if they’re fighting over coats, you’re taking all coats and jackets (and yes, they can go to school wrapped in a blanket and you don’t care what others say). You are confiscating chargers (and the phones/computers they attach to). You are confiscating hair accessories (and while you’re at it, brushes and conditioners and other luxuries). You are cancelling their cell and internet service, and using that money to pay for a lock on the basement/attic/guest room closet where all contested items will be stored for 2 weeks. You ar taking them off the car insurance policy, and they can ride the school bus with the younger kids, or they can walk (you will provide extra scarves and gloves since it is wintertime). To redeem items, they can choose from a serious “Job Jar” – clean toilets, empty trash/recycling, change oil in the car and add washer fluid, do the wash including sheets and towels for you and your husband, scrubbing the kitchen floor and the shower grout, wrapping insulation around the basement pipes, anything else you can think of. And you’re too tired/busy/tied up keeping lists to take them anywhere so there are no social events, no rides to the mall, no movie dates with friends. None. Every eye roll and “That’s not FAIR!!!” adds 2 days to the penalty period. And your calm, measured answer to everything is: “How unfortunate for you. I’m sure you’ll enjoy these privileges when you are older and more mature.”
Option 2: Do the above without 24 hours notice.
Re the trip: Don’t cancel yet, but don’t tell them this, but DO tell them what the cancellation fee is, and let them know you will be adding that to their bill. They can get part time jobs to cover it. They’ll have plenty of time to work (scooping ice cream, babysitting, shoveling driveways for neighbors, dog walking, anything they want….) since they aren’t going to have any privileges or clubs or activities. Also tell them that their jobs will have to cover the cost of a bonded, licensed, insured babysitter that you and Dad will hire to supervise them (through Care.com or any other service, or a retired police officer), while you and he go to Disney, and that you will be letting the police know that the parents are out of town and that an adult supervisor is on the premises. Tell them you need the week’s vacation without bickering or their rants about what’s so damn unfair from 2014 or whenever it was.
My son pulled some attitude on us during senior year. We took the plates off his car and made him ride the school bus. We took his phone. We told him we’d be calling the coach to say he was off the team. He capitulated in less than 48 hours. You have 2 kids and a longer period of insolence, so you might need 2 weeks IF you and your husband are totally aligned and do not cave in or negotiate. Tell the kids you will reassess in one month to see if these restrictions need to be extended. Maybe you can eliminate them in 2 weeks, but DO NOT tell them this! Do not appear to be willing to capitulate! Rehearse your speech with their father so you are entirely on the same page.
Have fun!!!