Have to say I agree with everything AprilMeadow has said. MN can be a great place for advice but there are always a few who seem to make everything so personal, and ruin the experience by being too free with the things that they say.
I think that some people seem to be forgetting that a c-section is a major opperation! I've never had one, and even I can see that! I was induced with dd and had a natural birth, but I couldn't have handled anybody except my mum being there after the birth. I felt like s**t, everything hurt and I could barely walk.
I think every woman has the right to be completely selfish after having a baby. See who you want/don't want etc. Your dsd doesn't have children of her own I'm guessing and therefore cannot understand what you will go through and how you will feel. So she wont understand that the last thing most people want is a room full of people after having a baby.
You inviting your parents is necassary, they're coming to look after your ds whilst you and dh are at the hospital. By the sounds of things, the very thought of her being there is already stressing you out. I don't think you're being at all unreasonable.
I think with her being a step daughter she will always feel an element of exclussion. It's not anybodys fault, that's just the way things usually are. Being 20 years old I think she should stop focussing on her wants (wanting to see the baby the second it's been born!) and start thinking about yours. How you will feel after having marjor surgery, tired, getting to know your new addition etc.
IMO, you're not excluding her or being unreasonable, you're in feckin recovery and not wanting an audience. I don't understand why some people can't see that.
Tbh, I think it sounds like your dsd needs to grow up a little bit, and your dh needs to support you. If it was him going through all this in 3 weeks time, I bet he'd have some different thoughts on the situation.