My mom won't get a job? Details below

That is a very valid concern. However if she is able to provide for you, your siblings and herself she may not have the need for a job. She may already have enough money to provide by other means, like an inheritance, insurance collections, lottery winnings,  beneficiary collections, high interest payouts or even stock earnings, as examples. Work is important but isn't always a necessity. 

Where is your father in all of this?  Is he in your life at all?  In order to know how to respond, we'd really need to hear your Mom's side of this.  If you are really concerned, go to another family member or other trusted adult.  Strangers on a website can't tell you what you should do.  You're a child .. She's the adult.  You can't tell her how to run her life.

As the others said, as long as she is financially capable of taking care of her children, she doesn't need a job.

Sounds like you may want her to get a job, just to get out of the house and "do something".  And that is a legitimate concern.  Even in the best relationships (including family), people need time apart.  If none of you have activities outside of staying home and going to school - perhaps you and your siblings should look into extracurricular activities.  (even if your mom doesn't have a car, there is always someone willing to provide a ride)

Go visit your father and relatives. School activities and get involved with your church. Volunteer at the hospital.