I had a chuckle when I read this ethical dilemma - not at you - but at me. You see - I am the sister with cancer, who does not communicate in any way with the woman who gave birth to me and I have a sister who sounds a lot like you - earnest, wanting to do the "right thing" and so on. Strongly influenced - I say brain washed - by a mainstream religious organisation she works for. Although not overseas as far as I know!
Long story short - I have cancer and it is up to me who I share information about my health, my disease and my prognosis.
I have made arrangements for what happens to me when I become incapacitated, as well as when I die, what happens to my remains and estate. I have been very responsible about this. I have an Advanced Healthcare Directive, Power of Attorney and Will all in place. With long standing friends who I trust to carry out my wishes to the letter.
I made a careful, deliberate and permanent decision not to have any contact with the woman who gave birth to me nor to share any information about me with her. There were sound reasons for my decision, which was made at a stage in my life where I was mature. I have not only never regretted it, but to the contrary remain happy about. Any regrets? Only that I waited as long as I did to make the break. The reason? She held my father hostage in mulptiplle ways and in order to have access to him, I had to go through and pacify her. His terminal illness and death liberated me from any need to see her ever again.
If I discovered a family member or friend who knew about my firm decision, decided to substitute their own views for mine, and pass my private health or indeed any other information to her, then I would realise I cannot trust that family member to respect my dearest and strongly held wishes. I would have to cease contact with a sister who did that too. Even if I liked or loved her.
I have to question my sisters motivation here. What does she expect will happen if she passes on my private health information? A Brady Bunch style reunion where we all hug each other and all problems are magically resolved prior to my death from cancer? Totally unrealistic but typical Pollyanna-ish approach my sister likes. Substituting your fantasies for how the world should be is not helpful.
Assuming the birth woman gives a damn, why the need to torture her emotionally by watching me fade and die when I chose not to be around her when I was well? If she does not give a damn, then telling her is unlikely to give her a personality transplant and make her care.
You know full well I don't want her told.
So, my sister - this is all about you, your emotional needs and specifically your unwillingness to carry any knowledge that causes you any inconvenience or, as it also possible given the total rupture between myself and the woman who gave birth to me, you wish to protect your own status with her as the 'dutiful daughter. And if she later finds out you knew and did not tell her, that maternal wrath that I chose not to have in my life will be 100% yours. I do not think you should ingratiate yourself further with the woman who gave birth to us as the expense of your dying sister. There is nothing she can do to help and much she is capable of doing to make my terminal period as miserable as she made my younger life.
I have a decent sized estate and no children. Your children will enjoy slabs of my estate - provided my wishes about this are respected. You see sister, I know you too well. I had to build an incentive in so - well, let's say - you remained motivated to respect my wishes. I saw you backslide as soon as that women launched into her routine with you. You don't love and fear me to the same extent you do her, so I hope the risk to your children's inheritance from me may be enough to sway you to keep quiet. Yes I know it's grubby, but left to your own devices you crumble like a Hoadley's bar without an incentive to rein you in.