Just go to poor families and you will see how early children develop into adults. You need to take care of your brother, or work, or both. Money makes a huge difference.*Not only that, but it is a major factor.
Having and fulfilling adult responsibilities does not make one an adult. Those kids are still kids, and the weight of those responsibilities on a psyche too young to deal with them, not to mention their own and as well as generational trauma, absolutely will catch up with them.