Do you mean if they can become citizens before the principal applicant? Because like rupeshhari said, they can. They can however not become PR's before the principal applicant. When the PR visas are issued to a family, They either have to land together and all become PR at the same time or the principal applicant lands first and the others later. The dependants can not land first without the principal applicant.
Once they become PR's there is no principal applicant anymore. They are all equally PR. If the former principal applicant leaves and loses his PR, it will not affect the others. They can stay and keep their PR's or apply for citizenship after 3 years.