I want to be able to work legally in the US. my mother is a US citizen, my sister is a green card holder. what do I need to do?

The fact that your mother is a United States citizen and your sister is a lawful permanent resident does not entitle you to employment authorization. You would need to obtain an immigration status that authorizes work. Your mother could petition for you to receive an immigrant visa. Once you become a lawful permanent resident, you would authorized to work in the country. You could also pursue a nonimmigrant visa that allows you to work such as an H-1B visa. You can read more about family immigration at http://myattorneyusa.com/family-immigration.