Harsh, but way to go Sara! It was the right move for the long term if anything serious is to develop between her and Ei. Ei may not realize it now, but when she looks back and reflects on this moment, she should see that Sara harbors a love and devotion to her runs more than skin deep; Being respectful to Ei and showing her she values her as a person first and foremost, and not just a mere sex object she lusts over, is admirable of Sara. It is a trait that any rational person would recognize as valuable in their friend, potentially making them someone worthy of building closer trust in, perhaps even seeing them as romantic relationship material.
RedMoonGod said:
Well...you snooze you lose Sara, you had a chance and you just had to go and fuck it up for yourself.
Rejecting the old doll is the logical choice that most likely should have left the door open for Sara to seek the companionship she clearly desires with Ei herself, should she try pursuing it. Accepting the doll would have only harmed her relations with Ei, both personal and professional; Ei wouldn't likely continue taking her seriously for long, and likely would have her doubts in her reliability in her high station as her army general, and probably would start looking for a replacement pretty quickly!
Sara now needs to collect herself to recognize and accept the prank for what it is and try not to dwell on it. Laugh it off with Ei, for "that damned fox" just set her up for her own personal amusement (as usual of her). Now to get her back somehow? Maybe discuss it with Ei to lighten the mood to move forward. Hopefully Sara doesn't just continue wallowing in grief for too long, lest she ends up painting herself as an easy target for Ei to pick on (and potentially one she bullies forever, knowing these sorts of stories), a scenario which takes less effort for Ei and rewards her with instant gratification over Sara's discomfort awkwardness, rather than trying to direct her attentions in ways that would work towards building something more constructive between the two.