Ursula and Teddy are the two bears of the Todd family (remember: Ursula's name means "little she-bear"(4.50)), and the two siblings are kindred spirits from the moment Teddy is born. Ursula often plays with him, helps him find his ball, and shares her cake with him. Ursula treats him more like a son than she does a baby brother. She would do anything to save Teddy, even kill another baby (well, if that baby were Hitler).
Besides her own deaths, the events in Ursula's lives that affect her the most are the events that hurt Teddy. In one timeline, he finds a dead body—but he and Ursula would "forever be the ones who found her" (20.174). When Nancy Shawcross is murdered, Teddy carries this grief around with him, too: "It's as if […] you walk into a room and your life ends but you keep on living" (20.600). In future lives, Ursula saves Nancy, less for Nancy's sake, we think, and more for Teddy's.
In one of the war timelines, Teddy takes Lucky the dog, who ends up not being that Lucky. Teddy disappears when his plane is shot down and is presumed dead. This devastates the entire family, but especially Ursula, who cannot do anything to stop this from happening. Although maybe that's why she shoots Hitler—after all, no World War II means no plane crash.
However, in another similar timeline, Teddy crashes, but survives. This is perhaps the defining moment in Ursula's lives, prompting her to realize that some events are out of her control, and she just has to live the life she's dealt the best she can.