Which would be faster to get my siblings into the US, if I, as a US citizen, or one of my parents, as a green card holder, petition for them?

A U.S. citizen may petition for a sibling in the Family-Based Fourth Preference visa category. There is a very long backlog for visas in that category - for a newly filed case it may take 15 - 20 years or even longer for a visa to become available (notwithstanding that the Visa Bulletin appears to indicate a shorter time), and so it is quite rare for that to be a desirable visa strategy. There are many details that can determine the amount of time it would take for a parent to sponsor a child. There really is no substitute for you and your family to consult with an immigration attorney who, after learning all of the relevant information, could advise about eligibilities, options, and strategies to attain your goals, and who then could offer legal representation in the often complex application process.