I'm the SD. Adult SS gets girlfriend pregnant. His baby is taken into emergency foster care with non-accidental injuries. There is no evidence to conclude who did it.
While the courts deliberate over the future of the baby, he is allowed to ask family/friends to look after it. If the court decide he is never to be allowed custody then this will be a permanent arrangement.
BM has been close to wits-end as you can imagine. Both of us have. BM is close to deciding that she will step forward and offer to take the baby. I don't want to. I don't want baby (strange statement for a SD I realise), don't want one, don't want anything to do with one. Don't want another 20yrs devoting my life to children. I want that time to share my life with BM. The time we never had without children.
If I refuse, she'll resent me forever.
If I agree, I'll resent her, the SS and the baby for ever. I won't even be the BF - I won't be able to bring it up the way I believe. The baby would always have dysfunctional Mum & Dad having contact and messing up it's mind.

I can't see there is a way out of this. I feel so sad, so resentful, and yes, so jealous. I always knew I would come second but I never expected this.