bio sibling health information? WWYD?

To make a VERY long story short, our AS is the youngest of four. By the time he was born (with significant special needs) the adoptive mom of his siblings did not fee like she could care for anymore. I have repeatedly pushed to see if AM would be open to some form of communication so that the siblings would AT LEAST be aware of each other. Several different CW's and adoption specialists have made remarks that they were doubtful this adoptive mom would be open to it. I bumped into a resource worker I am close to and asked again today who I should push to see if sibiling contact was possible. She gave me a contact person and disclosed that it would be good since our son's bio sister recently died from Stephens-Johnson syndrome. By bizarre set of coincidences, I already knew the entire story and horrible death his sister endured but did not make the connection that was his bio sister because names and family situation was never disclosed to me, I just thought it was some young girl who happened to be adopted. But let me reiterate, It was HORRIBLE! Our AS already has serious/anaphylactic allergies and other health issues. So, it's understandably frightening. There is some research that suggests there are some promising genetic markers that show predisposition to SJS. How would you go about pushing for health info if the AM is resistant? I wonder if I have any legal recourse? But I want to tread carefully and not throw my resource worker "under the bus" unnecessarily for disclosing too much though. So, WWYD?