It is difficult to answer your question because in your first sentence you mention that mom voluntarily gave up custody. Then, toward the end, you claim that mom never lost her rights. Operating under the assumption that mom never lost her rights, she is entitled to custody. You can try contacting CPS with your concerns, but unless the child's well-being is in jeopardy, I doubt that they will remove the child from her care. Just because a parent is struggling financially does not mean that it would be in a child's best interests to remove the child from home.
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I would suggest talking to the social worker assigned to your niece's case. Let him/her know about your concerns about the mother and about your willingness (and ability) to care for the child. If/when the mother is unable to do what she is supposed to, you will be right there to take over.
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