Adopting my deceased sisters new born kid with no fathers name ???

[RRindian]

We will be getting our green card soon..

Unfortunately my sister passed away 4 days back while giving birth to her new born kid.We have no clue who the kids father is. her case is still under investigation.

Our plan is to adopt her kid since the new born baby is no more having her mom nor no clue of who her father is..

So my question is

As a green card holder , can i adopt my new born niece who is just one week old as of today and is an orphan?

If yes please tell me what the procedures are because as per the website it says that to adopt a kid you have to be US citizen and can adopt kids based on green card..

Please please help me out..I dont want that kid to live a life of orphan while she has her own blood relative aunt to live with..