Thereby she can never have a child, thus fulfill her promise, that she will treat her husband's son as her own and by not having a child between them providies no opportunity for mistreatment of his son by a stepmother.
Thereby she can never have a child, thus fulfill her promise, that she will consider her husband's son as her own. She plans this idea as a last resort to prove her husband that she will never give room for mistreatment of his son by a stepmother.