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LW, take it from me ... there are few miseries on this earth as hellish as living with a step-parent who doesn't want you.
You can't help how you feel and don't let anyone guilt you out about it. Reality is what it is - you don't want to be her mother figure. It can't be faked - you will crack cause it's not what you want.
But worst of all, that poor girl will be seriously hurt if you come in there without wanting her in the family too. Please don't hurt that young girl like that. She'll pay for it for a lifetime.
And why torture yourself either?