Applying for Parents and siblings(under 21) greencard at same time

[muthvar1]

Hi experts,
I had a query regarding applying for Permanent Residency for my family. I will be eligible for citizenship application in Jan 2015. My sister will be 18yrs of age in the same year. I was wondering if I can apply for GC for both parents and sister at the same time. Or do I have to apply for parents first and then they apply for sis?