sponcoring family class

Hi evrerybody!
I am citizen of Canada since Desember,2005. I came in Canada as a refugee in 2001. When I applied to Permanent Residence from inside I declared my family members / wife and two sons under 18 years old / and I think so they were examined. Now I want to sponsor my older son /17 years old / who is student full time. After his coming in Canada as Permanent Resident I want to sponsor my wife and younger son /14 years old/.
My questions are:
1. Can I do so?
2.Should I incude wife and younger son ,as "non-accompaning dependants",in "Application to sponsor my older son" if I had already icluded them in my application to permanent residence?
2.If principle applicant are under 18 years old ,must he includes his mom and brother in application for Permanent residence as family members?
3.For single son under 18 his mom and brother are his family ?

For any answers,thaks a lot