The laws regarding child custody and adoption can vary significantly depending on the jurisdiction you are in. It's crucial to consult with a legal professional who is familiar with family law in your specific area for accurate advice.
If you haven't legally adopted your niece and her biological mother has parental rights, it is possible for her mother to have the right to take her away, especially if she decides to challenge the custody arrangement. Parental rights typically provide biological parents with certain legal rights and responsibilities regarding their children.
However, keep in mind that the situation might be influenced by various factors, such as the relationship between the child and her biological mother, the involvement of other family members, the child's own preferences (in some cases, once they reach a certain age, their preferences may be taken into account), and the laws in your jurisdiction.
If you are considering adopting your niece or have concerns about her custody, it is essential to seek legal advice promptly. An attorney can guide you through the process and help protect the best interests of the child while ensuring that you understand your rights and responsibilities as a potential adoptive parent.
Remember, legal matters concerning child custody and adoption can be complex and emotionally challenging, so it is essential to consult with a qualified legal professional to receive accurate and personalized advice based on your specific situation.