My wife just had twins and I have a 20 months old also. I work and I will need to bring my mother from Africa to help for about a month and half

Your mother must apply for a visitor's visa. She can start the process by filing Form DS-160. Your mother may face difficulty obtaining a visa. She will need to satisfy the consular official that her trip will be brief and she will return to her native country. Your mother should be prepared to show strong ties to get country.