WOW! What a selfish ***** of a birth mother!!! Would not give a vial of blood to help you with cancer??? And yet you're supposed to be GRATEFUL to her??? What, does her poop smell like roses, too?
Sorry, I'm the typical angry adoptee - birth parents bull**** pisses me off!
Contact your siblings, preferably by letter. Be honest, kind & respectful - of them. Tell them who you are, that you are aware that they do not know you exist, but that you have medical information which may affect them, and you do not know if 'mom' has shared that info. (since she was so forthcoming with you) and that you feel a need to make sure they are aware of it, and if they need anything in that department, you are more than willing to help & be available to them. Explain that you do not in any way wish to intrude on their lives, and you are willing to back off & leave them alone if they request it - and mean it. But that, since you are their sibling, you would very much like to meet them if they are willing. Stress the medical issue - it's non-threatening & may be easier for them deal with and use to 'justify' contact - with themselves or your birth mom. Take it from there, but take it slow - if bmom is any indication, this may not go completely smoothly.
Good luck & Goddess bless.