You HAVE been supportive but that doesn't mean you don't have any needs. These are family crises and they affect you as well as your husband and the kids. The job loss affects YOU financially and emotionally, and will also put more pressure on you next year in the form of increased hours. That will pull you away from the kids, the house, your husband, etc. Your MIL's illness affects you too (as the DIL, not just as the wife), as well as the kids (depending on their ages and level of awareness). They certainly feel their father's distance and preoccupation.
So I would say to address this as a series of FAMILY issues or MARRIAGE issues that affect more than just your husband. I agree that a long weekend would be fantastic for you, and if he's not working, he has the time to cope just as you have been. Go see your mom if she will pamper you, go see a girlfriend or an old roommate or a cousin - anyone at least 2 hours away. Don't just see it as a break for you - see it as a chance for your husband to really have some great bonding time with the kids and really be "present" for them. He's been preoccupied and distracted, understandably, but he needs them as much as they need him. His siblings can take care of their mom for that weekend. And I agree you shouldn't do all the work ahead of time like get the laundry done and make all the meals. Just GO. He can take the kids to the supermarket and run a few loads of clothes. It will even make him feel competent! One thing that happens when someone loses a job is that they feel rejected or useless or that they aren't contributing - you both need to see a dad-kid weekend as really ego-building.
Therapy is a wonderful option as well - it draws attention to your needs and helps you develop strategies for coping and moving forward. Again, he has the time to let you do this for yourself. Assuming he has medical benefits thru COBRA if you don't have benefits thru your part-time job, then visits will be covered. Ask friends or your physician for a referral to someone who accepts your insurance.
And lower your expectations in other areas - so what if the beds aren't made or the bookshelves aren't dusted?
Besides the fact that YOU need to be restored, aren't your husband and kids entitled to a wife/mom who is strong and rested and not depressed? You shouldn't need to freak out to get attention. There is a whole field called "respite care" which deals with the needs of caregivers (often the spouses or children of elderly or chronically ill people but also people in your circumstances) - they burn out trying to give give give. They need love and care too.
Be good to yourself. It's the best thing you can do for everyone concerned.