User blog comment:Xilinoc/Ch. 569 (You've been hit by / You've been struck by / Senbonzakura~)/@comment-24113610-20140223232155/@comment-8669536-20140224152023

It has actually been stated that Rukia was taken into the Kuchiki clan and joined the Gotei 13 49 years ago (this was revealed in chapter 179). She most probably either didn't recognise Isshin due to him having aged or she never met him because she was an unseated officer at the time.