foster to adopt question???

My husband and I have recently adopted 2 siblings thru CPS we did the legal risk, foster to adopt with our two babies the first time around. We decided that we were ready for another one and called our case worker. We have been placed with a 3 week old infant who we love dearly. His bio mom is currently in foster care herself and her parents parental rights have been terminated for some time now. My question is if anyone has been in our shoes about legal risk placement of a child whose minor parent is in fostercare herself and how did the whole situation work? We do not know a whole lot he was drug/ alcohol exposed but is doing well. His bio mom in her foster home is always running away, doing drugs, etc. So obviously not doing well to care for him. Please if anyone has been there email me or respond. thanks-rachel