"Hi neighbor, this is my cousin Kara. Her parents recently died in a car accident, so as her closest living relative in a position to provide for her, I've been made her legal guardian. She's moving in with me, and I'll be enrolling her in high school next week after she's settled in."
Okay, I'll admit, I'm biased against the whole "dumping her in an orphanage" thing having read PAD's excellent "Supergirl: Many Happy Returns" arc. There were editorial reasons for keeping Superman and Supergirl apart (the whole "secret weapon" thing, and Kara was supposed to be for girls to read), and it would look strange to people at the time if an bachelor was living with a teenage girl, but in-universe it just seems needlessly cruel of Superman.