She had been raped along with her friends from her village (who died from being raped), and saw her entire family (and village) hanged from a tree. Her new father figure killed by her other father figure. She also killed people to survive. Persevering through these horrible things must have given her the mental fortitude to withstand starvation.
Erik doesn't deserve Šárka to be his future wife... Even though you are besieged and starving to death, and madness consumes you, the ONLY people you don't take food away from are your wife and children. This means his instinct to survive and eat overwhelm his desire or instinct to marry and protect her. He's just a selfish boy who wants to marry her because she is a pretty girl. Not a good choice for a husband, really. Albeit that's understandable if you think about their age. Maybe he would grow into a fine man.