It's difficult to say if how your grandmother's dying was handled was correct and with her best interest at heart. Being told the truth does give you the opportunity to get your affairs in order and to say everything that maybe you haven't. Honestly, I don't know if I would want to know, and have to look at my loved ones knowing I will never see them again...it's a very personal thing. But as you said it's over and you can't change it now so it's best to accept that it was for the best. As for your mom, getting one's affairs in order does not necessarily mean one is ill or dying. We reach an age where we realize that it's time to make sure things are in place should we become ill. If she is ill, she probably feels she is protecting you and your sister by not telling you, not fair to the two of you but it would be about her at that time. I think what I would do is make sure you always let her know how much you love her so you never have any regrets for words left unsaid. I would also sit down and let her know that you really want to always know if she is sick and why. She may think she is doing what is best for you and your sister and by you telling her how you feel she may reconsider and let you know if and when something is wrong. We all say we'd do this and that until we're faced with our mortality and things change, so don't take her words too literally. Many of us have seen so much suffering prolonged with chemo and radiation that many are choosing to not endure all that goes along with this type of treatment and live out what time they have as healthy as possible, it's very common. Let your mother know that it has nothing to do with interfering but about love and wanting to be able to help her in any way possible. Let her know how you would feel if she were dying and never told you. If you find it difficult to talk to her then say it in a letter, but I think you do need to express your concern because it is really affecting you. Communication is imperative between a parent and child at any age and I've had to address my wishes with my children which wasn't an easy thing to do. As a parent we vow at your birth to always protect you and this continues regardless of your age. She loves you both very much and knows you love her, and I feel she thinks she is protecting you...which is what moms do. I know it's a very tough choice for you but it sounds like the three of you have needed to talk about this for a long time. I'm sure you want her to know that it's okay for her to tell you and she needs to hear this. Even if she denies that anything is wrong, at least you won't have the regret of not talking with her about it and she was aware of how you felt. Then regardless, everyday tell her you love her and get her to have lunch or dinner, do some fun things together, just spend more time with her and you will have no regrets. Big hugs to you and take care.
My mother died when I was 17, a few months later I was at a book store looking for a book in the suspense section and someone must have lazily placed this book on the wrong shelf instead of returning it to the proper place, called "motherless daughters" . I'm telling you it is absolutely vital no matter what age of your loss, or how it occurs. I've had several girlfriends read it when they've had their mom or have had there mom taken. All have gotten a comfort that we all agree we couldn't find anywhere else, be it, counseling, bereavement groups, friends , family or other self help books.
I seriously recommend getting it, you will find it comforting in any stage of loss