Asumming this is anything like Canon Moriya's family situation. Sanae was born from normal parents and then forwhatever reason just arent here anymore or we could just go with the usual "Business trip" excuse that gets used alot in manga. In other words, Sanae's actual parents just dumped her on Kanako.
Arael said: *looks at wiki* 3 and 7 year old girls?! Kanako is a serious lolicon.
What makes this doubly funny is that Kanako didn't have any benign reason to go to the Shichi-Go-San festival, either. She just literally went out to a festival for pre-pubescents with candy and marriage forms to go shopping for a wife.