Sister wants to give me legal guardinship of her 5year old son?

my husband is active duty military and we are stationed in okinawa japan. my younger sister is going through some tough times right now and is willing to sign over legal guardianship of her 5year old son to my husband and i. the biological father also agrees and is willing to let him come live with us but base legal is giving us the run around. how do i get this taken care of as quickly as possible so that i can have him back here in okinawa so he has time to adjust before he starts school starts after labor day. can she just sign a child guardian consent form with a notary or do we have to go through the courts. it's only for about two years to give her time to get herself back into a job and get herself straightened out