Indeed, Sophie is a complex character. There are many statements, actions, and representations that make her very unlikable. She does carry herself in a manner both during and after her experience at Auschwitz which can border on evoking a sense of disdain. However, the critical moment of her choice is where empathy, in even a dislikable individual, becomes the formative element in her judgment as a character. Having to make the choice as to which child will live and which one will die is the epitome of tragedy as a collision between equally desirable, but ultimately incompatible courses of action. Such a choice creates agonizing pain in any and all individuals. To have endured such a moment of emotional brutality makes anyone as a figure of empathy and sympathy. While Sophie is so very tormented and haunted by both the choices she made and the bonds which were broken by intolerance and hatred of self and others, she is a figure whose pathetic nature makes her one where it becomes very difficult to cast negative judgments and assessments.
Hang on, the only reason Sophie decides against joining the resistance is because she does not want to endager her children. Her motive is NOT Nazi sympathism. The reason why she turned against her father and husband was because she was mortified at their hatred for the Jews; that was what started it.
Also, he Sophie did choose to sacrifice her daughter BUT it wasn't cold or pre-mediated. The soldier suddenly started for both of the children and it just so happened the girl was easier to grab as she was in her mother's arms so Sophie just snapped.
I think that Sophie was without a doubt weak and made a mistake in getting involved with the Resistance in the first place (her connection to the freedom fighters featured in the film was also heart-wrenching as she did NOT help them yet got sent to the death camp anyway; I think she should at least have translated the documents for the group and perhaps achieved something for them) but why is your analysis of her so unsympathetic? She was neither an evil Jew-hater not an uncaring mother. She was put in an IMPOSSIBLE situation. Can you really put her in the same category as the real-life Jewish mother featured in This Way for The Gas Ladies and Gentlemen (for those who are unfamiliar with this work, the author describes in his memoires an incident when a young woman deliberately abandoned her six-year-old son on arrival in Auschwitz to better her chances of being selected for the queue which were sent to the worker's camp. Her child followed her, asking where she was going and why she'd left him and she still tried to get rid of him. When this drew the attention of one of the Nazi guards, he publicly declared her to be an example of Jewish swinery and then put them both on a van destined for the gas chamber).
I have no children myself but of the people I've asked who do, the general consesus as been that they would insist all three of them be sent off for gassing there and then rather than make that choice and live with the all-consuming guilt afterwards. But Sophie was not given much time to decide.
I think the other thing to remember is the surviving child was unlikely to survive the place anyway - it was, after all a death camp; fatal disease and starvation were rife and medical attention deprived. Those factors worked in conjunction with the gas chambers. It was a miracle Sophie herself survived.