Most of these lists are good advice.
Not sure that I'd carry a bunch of water. It's usually laying around in frozen form all winter here. A small supply is never a bad idea. A way to thaw the snow is imperative. Remember that you still need to purify the water if it is of a questionable source.
Basically, you need a sleeping bag or two and/or some blankets (depends on the size of your family) some ways to make fire, a nice first aid kit, signalling devices (whistle, mirror and flares are nice, maybe one of those distress signal flags and an emergency 2 way radio). Items to help you with improvisation (cord, wire, electrical tape, duct tape, etc) are always helpful.
Others have mentioned appropriate clothing but don't forget that headgear and gloves are a must. Get several. I like wool hats. A few waterproof/windproof jackets are nice also. I keep a few extra old work jackets with hoods in my cars. You could pickup stuff at goodwill. They don't have to be pretty.
The biggest threat in winter is simply getting stuck so I'd have
a fullsize shovel or two
bags of sand or kitty litter for the icy patches
Chain saw/ax/big saw is nice too for all those tree limbs or trees that fall once that heavy snow hits them and also to make cribbing if necessary
Tow chain/or big tow strap
Come-along and/or high lift jack
Ron Hood has a vehicle survival video that would be well worth the investment.
http://www.survival.com/volume-12.htm It concerns arid land, but the vehicle extraction stuff would be the same for you in a Maine winter.
I'm sure I'm forgetting something. Just my .02.