I suggest you look for a job in child care if you enjoy children. It can be a fulfilling life long adventure...lol.
If your mom and step dad are truly low income, less than $2500 per month your mom could qualify for reduced or even free child care through the state. I had one family that both parents worked a good job and each made over $10 per hour, I think it was more like $15 per hour each. They only paid a couple hundred dollars per month for a toddler, a pre-schooler, and an after school kid. If they had been paying cash it would have been a couple hundred per week easy.
I think that if you have any interest in child care that you could find work then your mom would have to learn to take care of herself and you could make enough to live on your own. This would show her you are ready to take care of a newborn.
By the way, it is against the law for any licensed child care setting to take a child under 6 weeks of age into their program. Because the infant is still under a doctors care from it's birth. Like your mom should not be allowed to go back to work for 6 weeks. She will be under a doc's care and if she were to get hurt on the job or start having complications from her birth experience they could be held liable even if she is the one that wanted to go back to work.
She should call, no, why don't you call your state labor board and ask them. Tell them that you have a question about leave after having a baby and if an employer can make a person come back to work only 2 weeks after having a baby.
IF they are threatening her with taking her job if she doesn't come back they could get in trouble. They could also be mean to her so it is best to not mention the company by name unless they are wanting to know and are going to stand by your mom. She needs that 6 weeks off to heal and bond with the baby.
BTW, there are a lot of ladies your age that are already mothers, some may even have 2-3 kids by 18. A hundred years ago my grandmother was only 20 and had 6 or 7. If we still had the tradition that girls didn't need their education and weren't good for anything but providing future work hands for the family farm then you'd already be married and have a few of your own. So your age has nothing to do with caring for an infant. It does have something to do with it if you have issues paying attention and remembering your responsibilities.
What if something happened to that child. Like SID's or RSV, Whooping Cough, something that could take it's life. Your mom would never forgive you or stop wondering if you did something wrong. She would have so much guilt and would not be able to stop herself from blaming you.
I still think that if you really want to take care of kids that finding employment in a child care setting would be the best route to go.