Well, there aren't any easy answers here. Your parents have put you in a very awkward position. They apparently are having a really bad time dealing with what's going on and are trying to tough it out alone, but what a shame that is, when you and your siblings could be there to help and offer comfort. Right now, for whatever reason, they're not looking at things at all from your point of view as a loving daughter.
I guess they don't understand what this is doing to you. I know I would never do this to my son and daughter, and I know my wife wouldn't let me do it, but we're all different.
I can't think of much else you can do other than to let them know that you love them, that you wish you could be there, that you would like to help in any way you can. Then, if worse comes to worst, you will at least know that you've expressed your love and that you've said goodbye as best you can.
I would have to say that in their pain and grief, they may not be thinking clearly, and I hope you'll take that into consideration, but I'm sure it's impossibly hard to deal with.