Where does the title "Plastic Neesan" come from? The title of Plastic Neesan is clearly refering to one of the characters and to the fact that they are doing plastic models. When looking at the anime, Neesan isn't more of a main character than Sakamaki or Okamoto. 

Is there any information about where this part of the title comes 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.