Dependent children should still be included in their parents PR application, either as accompanying or not-accompanying. That means if you sponsor your parents, you also have to show enough funds for their dependent children.
Children are considered dependent if they are single and under 22 when the application is received or if they are over 22 but full time students and financially dependent on their parents. In the former case, their age is locked in so they do not have to be students even if the application takes years but they must stay single. In the latter case, they must remain full time students without a break and financially dependent on their parents in order to keep their dependent status.
If children are not considered dependent, they can only be sponsored later under certain provinces provincial nominee programs, for example if the parents settle in Manitoba or Saskatchewan. In that case, you may also sponsor your adult siblings as long as you live in one of those provinces and meet the requirements of the program. Another option to help adult siblings immigrate would be to use your connections to try to help them get a job offer so they can immigrate on their own.