When to tell kept children about adopted sib

I need opinions. My b. dad had 3 kids after I was relinquished. He didn't know when or how to tell them they had an older sister that was given up. My b.mom had 2 kids. They have always known about me "being out there". Naturally I relate to both sides differently. Here's the problem. When b dad's oldest daughter was 12 b mom called his house looking for him and he was out. The 12 y.o. daughter answered the phone and ws told by the b mom that her dad was her old b friend who had kid with her and gave it up for adoption 16 years earlier. This sister now suffers from PTSD because of this experience, it fractured her relationship with her dad, it strains her relationship with me. I want this sister to heal from this experience and I think my b. mom should have some concern and remorse. She says that he should have told his daughter like she told hers and that it's really his fault she got hurt because b. mom didn't know his daughter was in the dark. however the sister says b.mom told her "well obviously your father didn't tell you about.." years ago when she called. it hurts that b mom's daughters defend her. How do we resolve this?