Abbey, you seem to forget this is the young man who has gotten into accidents in his mother's car; took off burning rubber (or close to it) with his mother and sister in the car; has hours of community service to do because he can't get out of bed in the morning. This is a easy child with a total attitude in some areas.
I'm not a big fan of curfews but if a kid has proven he can't be responsible enough to get his rear into bed at a reasonable time so that he can get up in the morning, he needs someone to make those decisions for him.
What's hard here is that there is a track record of things changing for a day or two. Mom calms down, all is good and then things go right back to the way they were. The kids are totally taking advantage of the fact Heather is sick. Rather than rallying and trying to help more, they instead act selfishly, are inconsiderate and blame her illness for them not getting what they want in life.
Heather, I wish I could magically wave a wand and makes things better. I can't. You need to decide how many battles you can tolerate and then make a plan from that viewpoint. Any consequences you give have to be followed through consistently and always -- not just a day or two. You have to make the rules and you have to make them stick. It's hard work. It's tiring. It's no fun.
The consequences have to be real and they have to be fair. For example, no food until previous dishes have been rinsed and put in dishwasher. You're 3 minutes late on curfew, that's 90 minutes home earlier tomorrow night (30 minutes for each minute). Until you keep your temper in check, no using the car unless it is for the family. That means you find your own way to school, to your girlfriend's, to work, to community service. When you're asked to do something, it should be done within 30 minutes with no attitude. Stomping, storming, yelling, calling Mom names means Mom doesn't do for you for the next day -- no taking you anywhere, no computer time, no TV (Mom pays the bills for all of these, no reason for you to enjoy them when you won't help out the family).
A list of chores for everyone and by when they must be completed might help a little. The list could also include the consequences of them not being done.
Anyway, those are my suggestions. The main thing is you need to do something and it's need to be real to the kids.