In My Ántonia, Yulka is Ántonia's younger sister, who we learn is mild and obedient. She is a minor character, and we don't even hear her speak. One of the important things she does is demonstrate that Ántonia is a naturally motherly figure. Ántonia always takes care of her sister, both before she leaves the farm and after. When their father dies, for example, Ántonia comforts her sister at the funeral. Later, one of Ántonia's motivations for working in town is to provide for her little sister. When she is with Lena by the river, for example, she asks for her shoes to give to Yulka.
After Jim is done teaching Ántonia how to speak English, he starts teaching Yulka. This is important because it reiterates how the younger generation of immigrants ends up adapting well to the new culture and even helping their parents with things like language. Still, because she is the youngest, Yulka doesn't bear the same responsibility that Ántonia does for working and supporting the family – this is another insight into the immigrant family structure.