This is a complicated process, which would have to meet the requirements of the Hague Convention (on international adoptions), which involves specific steps for each country (China and the US). It would be best that you talk with an immigration lawyer familiar with international adoptions. Feel free to contact us at www.adoptfla.com for a free initial consultation.
At this stage it might be much easier to do a temporary custody by extended family, and then an adoption once she turns 18. All you need is her father's consent to the temporary custody action and this type of action is relatively inexpensive, depending upon who you hire to do it. You will have to seek the advice of an immigration attorney to find out about that aspect of what you want to do, and your niece becoming a US citizen.
R. Jason de Groot, Esq. We do not have an attorney-client relationship. I am not your lawyer. The statements I make do not constitute legal advice. Any statements made by me are based upon the limited facts you have presented, and under the premise that you will consult with a local attorney. This is not an attempt to solicit business. This disclaimer is in addition to any disclaimers that this website has made. I am only licensed in Florida.