^ Dia found out that Yoshiko is dating her sister Ruby. She immediately forbids Yoshiko to go out with Ruby any further, but Ruby intervened and stated that "Well, if that's the case, then why are you dating your best friend?". And with that, Dia was defeated and allowed Ruby and Yoshiko's relationship to go on.
Wait hang on, something is wrong in the above paragraph.
but Ruby intervened and stated that "Well, if that's the case, then why are you dating your best friend?
Yeah, this line here, this part in particular:
then why are you dating your best friend?
Well, Maru seems to have supplanted Mari and Kanan as Dia's best friend, that still wouldn't make Dia give Yoshiko instant approval, since Ruby has nothing to hold ovef Dia otherwise.