Our situation: Mom lost son#1 to couple in Maryland. We adopted son#2. At the adoption, she was pregnant with son#3. DCS took son#3 and he was adopted with son#1.
Then she had SON#4!, but a different county and she KEPT him. A few services for the baby (PT, and like an early intervention program), but nothing for mom
. BUT, I maintained a relationship with bio-mom, we do occasional visits, and she let us have the baby (son #4) for the weekend!!! She'd let us have him more, but I prefer things how they are. I do have to say that the weekend we had him(he was 15mo), he was clean, no diaper rash or runny nose, no bruises, she packed plenty of diapers, clean clothes, clean bottles, a light jacket (it was summer) and he seemed developmentally on target.
BOTTOM LINE: Unfortunately, you have NO rights with this baby. I'm sure if things don't work out with mom where she is, that you would be in line to foster/adopt, but as a previous poster said, new dad's bio family are also now considered resources as well.
MY ADVICE?? Put aside any personal differences, and get friendly with bio mom. Establish a relationship with her. Do visits with her and your son and the new baby so the kids can get to know each other. If things go bad between her and her mother-in-law (and I believe that they are bound to), she would consider you a resource for the baby.
BEST CASE SCENARIO: Mom improves her life, becomes a responsible parent to new baby and your son has a relationship with his bio-sis.
WORST CASE SCENARIO: Baby comes into care for whatever reason, and you can argue for placement with you because bro and sis have an established relationship.