PLEASE be gentle when you bring this up to her. Its very scary to face the facts, especially when those facts take away your happy world, your trusting world. I was that girl once. Everyone told me after my CS, "Oh, you're healthy, the baby is healthy, you should be happy." Well I wanted to be happy, and I kept trying to stay happy, even after I found out I'd been lied to, and that my entire reproductive life will be an uphill battle from now on. Every time I came up with a fact, or a part of my plan, or told someone anything, people in my life would have basically one of two reactions: "Be happy, healthy baby, healthy mom" or "I feel like hospitals are evil you shoiuld just flat out do what I would." Its my real life, its complicated. What I want so desperately may not happen no matter what I do. I can't even tell you for sure that I'll still be pregnant in my freaking due MONTH, how can I tell you what that life-giving moment will be like?
It took me months to get to the place I'm in, and I only have one CS to deal with. It was alot of gentle suggestions and God-thing timing, alot of I-think-I-wants that hit the hard wall of reality in the quiet of a car ride home. Even if you convince her (and yes I have a relative who will be sure she convinced me) she might not tell you for a very long time. Please, please, be judicious with your words. You have no idea how hard this can be. She's not pregnant yet, I take it. I didn't tell people we were trying, in case it didn't happen, because I couldn't know, and people put alot of pressure on you to KNOW.
PS I've heard of VBA7C, and of 11 CSs. I also have two friends with 4 and 6 children respectively, all CS, who can't have any more children. Physically can't even conceive. Life is complicated, unlike the internet where we're all anonymous good hearted folk with lots of opinions!