Where does the title "Plastic Neesan" come from?
The title is just referencing the main character Iroe Genma. Due to her status as the head of the model club and a third year high school student the underclassmen call her Nee-san. It is true that Sakamaki and Okamoto are also main characters but in every synopsis of the manga and anime that I read stated something along the lines of:


  Plastic Nee-san is a very short anime that follows a third-year high school girl who 
  likes building plastic models and the wacky conversations she has with her fellow club members.