The way you described your reaction to your mother's and step fathr's comments, it appears indeed that you have not yet reached maturity. Perhaps it helps to create a 'battle plan' and write down risks of pregnancy and raising your child for you, as well as how you intend to address potential problems in the future. Maybe your family can elaborate more what their worries are and you may be able to relate a bit to their side.
Since you want to be a mother, please put yoursel in your mother's shoes. Try to understand why she is worried at the prospect of you having a child. Try to find what the best for your baby would be; perhaps you will cone to the conclusion that you might want to wait a while till you are ready. Also talk to your family about why you want a baby and why necessarily now? What are your expectations?
A child can change people but bringing a child into this world - who depends on you for everything- and hoping his or her existence will bring positie changes is not the right way to go. Even caring for a pet is a huge challenge when you have health issues and/or you are not ready yet. Parenthood is often glamorized by starlets in the media but for most people it is messy and exhausting and very expensive. Healthy and otherwise capable people are sometimes pushed to the limits due to sleepless nights and all the stress a newborn brings. So Id say the bare minimum is to be physically and mentally able to care for a baby, be prepared to put your needs behind your baby and famoly's needs, and ideally have a stable partnership to support you.