Adopting my wife's niece... (From another country)?

My wife and i want to adopt her 8 year old niece that currently lives in Ecuador and bring her to live with us in USA. My wife is originally from Ecuador and is now here with a perminant residents card from getting married with me a natural US citizen. Her niece is currently living with my wife's mother because her actual parents dont take care of her and financially wouldnt be able to. My wife and I both have good careers, we own property in California and have a stable lifestyle. How could we do this????