Yes you are living in an impoverished state of existence.
You getting a job will help augment the family income, yes.
However, you are rich in the desire to succeed, and I can see by your excellent grammar, that you are an incredible student. You are going to go far and be successful. True executive material.
Maybe even run Yahoo one day.
Yes you guys are living within poverty. Your step father should get a job. If he is handicapped then he should go to his local social security office. You have a couple interviews coming up, so if you get a job, it will only help your mother's income if you share but your main focus should be school. I seen and heard a lot of kids say that they will go back but never do so please focus on an education(degree or short term certification) or you will be within your mother situation..
Best of luck.
Officially, yes. Your quality of life also depends which part of the country you live in and how your family lives. While you are in school, avoid student loans. Student loan debt will reduce your income to poverty levels.
By any standards I can think of, yes.
The last time I tried to live on a similar wage, it was pure hell. Good luck on the school thing, I hope you are getting some scholarships.