Ann is alone. In her own words, she says she has "turned away" from her father. While he is in prison, she refuses to visit or to write. When her brother George comes to the Keller home in Act II, Ann chooses to remain with Chris instead of leaving with George as he asks (and demands). She has no family of her own and so is tied to Chris.
This is a fact of her own perspective, so to speak, and a fact of her decisions and we see this clearly in Act III.
When Chris re-appears, he tells Ann that he thought she would be gone. She replies that she has nowhere to go.
We learn in this act that Ann is capable of forgiveness, to some degree, when she reveals the letter that she has been holding on to from Larry.
...she carries what is in fact a suicide letter that Larry wrote to her before his final mission.
Knowing that Larry blamed his father and that, as a result, Keller is responsible for Larry's death, Ann is still capable of forgiving Keller or at least speaking with him. She will not speak with her father, but she needs some family.
This necessity leads to a complexity of character in Ann that is hinted at in the earlier acts but which becomes fully articulated here in the final act.
Ann is not driven to hate or revenge. Rather she is seeking healing and feels that she can find it with Chris and, therefore, accepts Keller and looks for acceptance from him. Her isolation leads her to this.