Hi Hupsy
Hopefully you are on a plane home and this will be a moot point...
However-if you are still trying to fill your days, I am on Long island and can offer a few suggestions to keep you out of the house. Not sure how old your DC are, but hopefully some of these places might suit? Most are on the North shore as that's where I am :) but it's really not far by car.
Bayville Fun Park in Glen Cove (north shore-about 30 mins from Long Beach) is pretty good for little ones migraine inducing for adults, take earplugs(mini golf, climbing area, water squirty boats etc)-beach right over the road. Vanderbilt planetarium in Centerport has celestial kid friendly shows and a nice park and the Vanderbilt house (might be a bit boring for the kids)plus lovely little beaches all around. if you want a bit of open space to run the kids around, Planting Fields Arboretum in Oyster Bay is lovely-has good café etc. Depending how old your DC are, there are several canoe/ paddleboard places nearby, and the water is much calmer oop north. Several good swimming pools in the state parks, or Ice Skating rinks if you really want to cool off :)
If you drive to Port Jefferson, you could get the ferry to Connecticut and drive to Mystic Seaport (old harbor/dockyard/boats). Ferry is (I think) about $60, but best to check :) takes about 40 mins to cross. Long Island Children's Museum is supposed to be good, but I haven't been for 10years or more. Planetarium and Children's Museum in Lower Manhattan also excellent. Plus you can go for fish and chips at A Salt and Battery :)
I'm in Northport on the North Shore-would be happy to offer a healing cup of builders' tea. I've got chocolate digestives and everything (grin) PM me if you'd like to meet up.
Your DSis and BIL sound beyond awful. Hope you've managed to enjoy some of your stay.