This is a quagmire. You are willing to break the JW rules, but the person receiving the gift is not. If you told the doctors in secret that you are willing to have blood, should it be needed, they still know your sibling is not willing. You leave them in a bad place- "try not to give you blood, and don't say anything if you do have to, and yet still risk the recipient's life."
If your sibling is not 100% with you, willing to just shut out other people from knowing that either of you will take blood if needed, then I would suggest you just walk away from it. I wouldn't even bother to find out if I was compatible, as that just gives your sibling more to report to the elders- that you went so far in pressuring them as to even see if it could be done. I might do it on the low down- ask the doctors to determine if I am compatible without telling my sibling, fully explaining the stupid JW thing. Your sibling could change their mind, but I doubt it.
I know a life is involved, but it's their choice.