Crowning Music of Awesome: The show's opening theme "Omoidatta ga Kichijitsu", performed by Tokugaki Tomoko, Iwanaka Masako and Inabe Kumiko, who are collectively known as...guess what? Three Sisters! Full version with lyric here .
There's even an English version known as "Today is the Day". 
Wait till your heads go "Tilt", because here come the Three Sisters as in the show!