You need to speak with an adoption attorney ASAP. (You can start with JAG, but often those attorneys do not have enough experience with adoption, so you will probably need to find a non-military practitioner. Check the American Academy of Adoption Attorneys website for recommendations.) Among other things, even if the father is "unknown," his rights still need to be dealt with under the applicable state law. You also need a homestudy even though the birthmother is your niece. This is do-able, but you need to get an attorney on board right away. Good luck!