1.) How does one say NO to a single mother with three kids, all under five, when she needs someone to watch them so that she may provide food, shelter and clothing for her family?

Saying "no" is likely one of the reasons you find yourself in such mental torment all the time...just takes one who can't say no to know one :-) You didn't marry your sister..., for you to help when you can is noble, but not at the sacrafice of your own health. I think putting boundaries on the helping is healthy..both for you and your sister...devise alternative solutions for the problem. I am sure your sisiter knows this is not a permanent situation for you, don't let it become one, then have resentment build that would make you even sicker and be unfair to your sister. Yoursister will figure it out.
2.) How does one say NO to a teenager when her life is falling apart, and her own parents are only contributing to the emotional stress, and you have been the Aunt that has always been there?

OMG teenagers need "NO" in their lives...again are you seeing a patteren with the "no" thing? You are putting yourself in the situation of keeper, when you need not be. I know it is tough, and people will have to adjust to you saying "no" but help them with finding alternatives other than relying on you, again, if they knew how this was hurting your mental health, i would think they would want you to be well.

3.) How do you not get mad or upset for your needs seem to be left behind.... even though this has been the nature of the beast for almost ever? I am thinking you would benefit from a book called "Co-Dependent No More" Again, I am seeing a pattern, only cause I have been there. Trust me people will get along, and you will not be breaking down on the inside cause you feel like your needs are not being met.

4.) How does one find their own SANITY, and keep it, when others need / want your help.

You say "no", that is how you keep your sanity. You listen to the voice that pops up and says "i don't want to do this" and instead you hear your own voice say "sure I cann help" Learn to set boundaries, people will not stop loving you even though they may not like being told no, in fact you and I really don't like to hera no either. You are totally in control of all these questions....good luck with them, and may God give you strength to say "no" when needed.