It seems to me that Sato was never in love with Shiratori at any time. This is probably why, for instance, she never accepted Shiratori's offers to dine out at high-scale restaurants. Additionally, during Sato's forced marriage, she acknowledged how important Takagi was to her just moments before she was about to kiss Shiratori (which, as we all know, was interrupted at the last minute). The only time Sato actually loved someone else was during episodes 301-302 and 304, when she fell in love with Matsuda, but Matsuda died during the apartment bombing case. I think this is part of the reason why, after breaking up with Takagi in 302, Sato had to take up Takagi once again by the end of 304.