The weekend I went to see my parents to tell them that dh and I were going to have fertility treatment, my sister turned up to tell them that she was pregnant. I was 2 years older than her, and had been trying for over a year. She had been trying for 3 months.
6 weeks later, she found out it was twins.
My world just fell apart. I could cope with most people being pregnant, but not my sister. I felt wracked with guilt about how much I resented her, but couldn't bear to see her when she was pregnant. I didn't want to hear her talking about being pregnant, or worse still, complaining about being pregnant and feeling sick/tired, when it was something I wanted to feel more than anything in the world.
Things got a little easier once she'd had her babies. 6 months later I found out I was pregnant. To my sister's enormous credit, she never, ever held against me the fact that I had kept completely away from her while she was pregnant. As my sister, she just understood how awful it had been for me, not being able to conceive, and loved me enough just to be overwhelminghly glad that we eventually had her baby.
I suppose what I'm trying to say that unless you have suffered from infertility, you can never really understand how it tears your heart out when you hear about other people being pregnant, or when you know that the only way you will have a child, or subsequent children is with major medical intervention. You will never know the unexpected joy and realisation that your period is late. You have no control over the way things happen.
Your sister having a cs may be her way of having some control. She will need a huge amount of support once she's had the baby too. She may have spent years imagining what it will be like when she's a mummy, and it's unlikely that real life will match up.
I hope that, like us, once your sister has had the baby, she will start to see the similarities again between you. Until then, you just need to give her your love and support and understand that whilst no more loved than your child, her child is far harder won.