Airplane with a different family? and im 13?

so im going to travel with my sister's friends and theyre a family of 3... and im 13.... so what do i need to be able to travel with them? so i need like my student id? or a signed note from my parents? oorrr what? do i even need anything? cause people think 13 is like really underaged andstuff (but im almost 14 :) ) and i am not going with anyone im related to so will i notbe allowed to goo? im leaving early next week so plleeeaaase help... thanks!