I am not sure of a anime I have or seen but here how it goes.
She is elementary school but has the looks and body of a high school girl and her cousin he is high school but she is tall just like him. And her living with him makes it a odd living situation. He finds her attractive but she is still elementary student and he is high school student. It's not Cyclops Shoujo Saipu. But the characters are similiar drawn.