J.C.
M.,
You are definitely taking on too much. You must ask for help (with watching the kids, or figure out how to stay home with them full time). I don't know the situation with your niece & nephew, but is there any way you could qualify for funds to help take care of them. I know foster parents get a monthly stipend. If you are keeping the children from going into a foster care situation and could prove it, surely there is a government program that can help you. If not there, if you attend church, they may have a program too. At least some assistance until you can financially budget for the extra 2 mouths.
What part of town do you live in? There may be a non-profit group in your area, or maybe myself or other mom's on this board that live near enough to help out.
For sleeping (you could try listening to a sleep meditation recordings). They work like a charm for me. You can download free podcasts on the interent (apple.com downloadable MP3 podcasts for free).
My heart goes out to you for taking on so much and at such a young age, but you do need to take care of yourself first (or there will be nothing left to give the kids).
Let us know where we can help you.
J.