I'm always asking myself a similar question. How can the author draw such beautiful boobs but the rest of their body looks like crap. For example: previous chapter page 10. Her butt and thighs look so bad like if he/she draw them for the first time.
@Loldarkking I also don't see a problem if they will be together after she turns 18.
If Yoshida really likes to live with her then that means that they are compatible. There isn't anything better then compatibility. Also Sayu would make a great wife.