Probably not at the same time

The short answer is that you (when you become a citizen) can apply for your parents' immigration without having to wait for a visa number. The whole process can be finised in approximately 1 year. You (when you become a citizen) can also apply for your step sister's immigration, which will be subject to the number of visas available. The current waiting time for bro-ssi category is about 6 years.

In the alternative, your parents, when they get the greencard, they can apply to bring their child to the US --- this will be faster than your application for your sister.

It is unlikely that your parents and your sister can get the immigrant visa at the same time. However, they do not have to be separated for any significant period of time because your parents can go back to China as soon as they have received their greencard and wait for your sister's paperwork.