User blog comment:Dragonzako/Thoughts on Luna Loud/@comment-27245645-20171222055237/@comment-30498740-20171222161804

@Entertainment  To answer your question, its because she was the first sister to have an indiivual bonding episode with Linc (For Bros About to Rock) and since the episode was so good and two didn't get into a fight the special bond came out and then that evolved into the dumb gurdian sister trope.