Think of a problem for the sisters. Because it's the two of them it would be better for the story if the problem included both of them rather than one. So one doesn't just sit there like a sounding board. Doesn't have to be a man or anything romantic it could be a work related problem, something financial, or unethical - one sister stole the other sisters identity and ran up some bills etc. Or something from the past. Just think about your characters what do they do, what do they like, what are their interests, their fears. Something should come up.
I'd stress: cancer in itself could be a story. To address it as "get it, die/don't die" is like addressing the human experience as "have it, deal with it/don't." Sure, maybe some plot wouldn't be factoring in, but there is plenty of conflict in dealing with cancer, especially since one could begin to address philosophical questions, due to the suffering of life and the nearness of death. What is the goal of this radio play?
Sorry, but you are asking for help with the one thing that MUST be your own. Anything we tell you will be our idea, not yours, and we will know it better than you ever can. Your idea must be yours, and you must know it better than anyone. Forget the radio play. Forget what you hope it becomes. You've put the cart before the horse. You need to develop the story. What is the essential truth you want to convey to the rest of us? What do you know about these two sisters that we don't know and will be better off when we do? Once you know the essential truth, take a good look at where they are right now. What are the possibilities? Which possibilities do their personalities make most likely? What choices do they make? And what strokes - for good or for ill - might fate deal them? And, finally, where does it all end? Once you have all that, you can decide how it becomes a radio play. And then you can worry about how much time you have and how much you need and what to cut and where to fill. But first, you need the story, and that has to come from within. Good luck.
I think @EdFromNY is right. This has to come from you, not us out here. And I also agree about getting the cart before the horse. Don't worry about having 8 minutes' worth of dialogue in a radio play, when you haven't a clue yet what the play is about. Unless this is some sort of assignment? Write a radio play? If this is the case, you really do need to change gears. If the idea of the sisters and cancer isn't coming together for you, then I'd drop it for now and come up with another topic to fulfill the assignment. Write a radio play about two sisters living together in a house that's falling down around their ears—one of them is happy to deteriorate along with the house, while the other one wants to get it completely renovated. Or a story where one elderly sister resents another one because of some old love triangle, or what they remember as favoritism from a particular parent? Or one of them pretends that the third one who died is still alive. Or each blames the other one for the third sister's death. And it's not either of their faults, really. Or it's both of their faults. Or only one of their faults. Or one sister is getting married and the other one feels totally abandoned and resentful because she can't or won't get married herself. Or she is already married, but her marriage is falling apart. She is jealous of her sister, and either openly discourages her sister from getting married because of her own bad experience, or takes the catty option of sowing seeds of doubt to spoil her sister's happiness while pretending to be supportive. Or maybe they aren't sisters at all, but are living together and pretending to be so that people 'out there' won't realise they are actually a lesbian couple—and now one of the two is sick of the pretense and wants to make it clear to the world what their relationship is, but the other one is frightened of the consequences. You see how to do this? Just start throwing out every idea you can. Keep hacking away. Sooner or later one of the ideas will click, and you'll start to explore it, and MORE ideas will click ...and before you know it, you'll be writing like mad, trying to keep up. What you should never do is rely on the forum to generate your story for you. It's like saying "Hey, I want to make up stories—make one up for me, and I'll write it down." We can help in many ways, but the urge to write and the stories you want to tell have GOT to come from you. You are unique, with your own perspective. So don't be afraid to let your mind generate your own ideas. There is nothing more thrilling than creating your own fictional world. Don't worry at this stage what other people will think. Just get it out there and get it down. If you run into a snag, stick with it till YOU figure it out. Go for a long walk to clear your head, look around you, and see what you see. Talk to yourself, and scribble down ideas until you get your eureka moment. And good luck!
I was once in a short radio play that centered on an abuse victim finally voicing the pain the experience caused. 8 minutes isn't a very long time, so maybe you could focus on one emotion, like anger. For example, the healthy sister wants her to be happy (because she didn't die) but the other sister can't move on just yet. This sort of tug-o-war could also bring other issues to the surface between the two of them. I also agree with EdFromNY, but I hope you find your inspiration.
I think @@EdFromNY is right. This has to come from you, not us out here. And I also agree about getting the cart before the horse. Don't worry about having 8 minutes' worth of dialogue in a radio play, when you haven't a clue yet what the play is about. Unless this is some sort of assignment? Write a radio play? If this is the case, you really do need to change gears. Just thinking --- i had no idea where i was going with my full length stage play until it happened... i kept writing and getting feedback until i got to the end - then I went over it and haven't yet finished editing it... it has changed a lot and has got a lot better... but it is still a full story... i just didn't plan it until I worked through it bit by bit. I know some people plan their stories, but i'm not sure if I am like that
We can help in many ways, but the urge to write and the stories you want to tell have GOT to come from you. You are unique, with your own perspective. So don't be afraid to let your mind generate your own ideas. There is nothing more thrilling than creating your own fictional world. Don't worry at this stage what other people will think. Just get it out there and get it down. If you run into a snag, stick with it till YOU figure it out. Go for a long walk to clear your head, look around you, and see what you see. Talk to yourself, and scribble down ideas until you get your eureka moment. And good luck! my problem is that i can't seem to stop writing autobiographical theatre... and although i have lead a full life - this is a really exhaustible source!!!!!
Think about what you are trying to gain from telling this story. How do you want your audience to feel? Is this a reality check or something to uplift and find hope in?