I am a citizen, is it better for my parents (who are not yet permanent residents) to file for my sister or for me to file for her

You and your parents can both file for you sister. Visa availability times are changeable and the time for sibling petitions may speed up in the future. Also your sister's circumstances may change, e.g. her getting married, which may necessitate a backup plan. Neither application is complex where there are common parents.  Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.