help!

My fiancee and I recently found out that his sister is going to have a baby girl in September. Shes a heavy drug user and told us that most likely CPS will remove the baby once she delivers. My fiancee and I already have a one year old son, but we would really like the chance to adopt her daughter. We don't want her to end up in foster care, when we have the extra room and love for another child. Has anyone been through this process before? We have no idea how to go about getting custody once CPS steps in. Any advice would be great. We're getting married in July. He works two jobs and I work full time but I would be staying home if we were to adopt.