The stepmother of Buddha was Mahapajapati Gotami who looked after Buddha after his mother passed away shortly after childbirth. She attained stream entry (first stage of enlightenment) after listening to a discourse given by Buddha when he came back to his hometown Kapilavatthu.
King Suddhodana (Buddha's father) passed away at the 5th year of Buddha's ministry as an Arahant after listening to discourses given by Buddha. Then Mahapajapati wanted to join the order too after his passing. She made the request to join the sangha 3 times but was refused by Buddha.
She was persistent and with 500 other sakyan ladies shaved off their hair, donned on saffron robes and walked barefooted all the way to Versales where Buddha had gone. Venerable Ananada who was a new young monk back then saw their pitiful state and empathize with them. So he approached Buddha on their behalf to asked for their ordination but was refused thrice. Then he asked Buddha if women can achieve enlightenment too? Buddha was affirmative that they could attain enlightenment too. That opened the doors to the formation of the Bhikkhuni order which flourished for the next 40 years during which many Bhikkhunis (nuns) were enlightened.
In the Mahaparinibbana Sutta which was given by Buddha during his last days, Buddha only passed on after he was assured that four groups of people namely the Bhikkhus (monks), Bhikkhunis (nuns), male lay disciples and female lay disciples could exhort and expound the dhamma well.
It was said that in order for the nuns order to be formed the 8 garudhadhamma (strict rules) must be accepted by Mahapajapati Gotami first. These 8 rules are :
1. A nun, who is ordained even for a century, must greet respectfully, rise up from her seat, salute with joined palms, do proper homage to a monk ordained but one day. 2. A nun must not spend the rain retreat in a residence where there are no monks. 3. Every fortnightly, a nun must request from the monks, the Observance Day and formal confessional meetings. 4. After the rainy retreat, a nun must invite both orders and confess to any offences. 5. A nun, on committing an offence must undergo disciplinary action before both orders. 6. A novice has to be trained for 2 years before she could seek higher ordination from both orders. 7. A monk must not be abused or reviled in any way by a nun. 8. Admonition (reprimanding) of monks by nuns is forbidden. Admonition of nuns by monks is not forbidden.
But these were likely later scholars' inserts as some may not be consistent with teachings of Buddha. Another possible inserts found in Gotami sutta mentioned that a prediction was made : the dhamma would last 500 years if the nuns order were to be introduced. This is again inconsistent with Buddha's teachings as he does not encourage fortune telling/predictions; he emphasized on anicca (impermanence), dukkha (suffering) and Anatta ( non-self) which are three characteristics of existence. Buddha said that so long as there is noble eightfold path, the Dhamma will be alive for a long time. Dhamma is alive still although 2550 years had passed after the first formation of the bhikkhuni order.
In AN 8.53, it was mentioned that Buddha gave Mahapajapati Gotami a short discourse to help her to practice and realize the dhamma. She was told to discern things which lead to:
Dispassion and not to passion
Detachment and not to attachment and bondage
Dismantling and not to building up
Fewness of desires and not to strong desires
Contenment and not to non-contentment
Easy rather than difficult to support
Solitude rather than company
Arousing if energy and not to laziness
At her final passing, Mahapajapati Gotami was exhorted to display her powers to show men who did not believe that women could not equal men in terms of wisdom. Apparently she displayed the twin miracles of fire and water and her asked were then collected in a bowl.