Eldest Daughter problems, advice needed..

Whoa, wrong approach Optimi. A child shouldn't have to feel guilty because you had a difficult birth and a C section.

What I've just messaged her in an attempt to sort things out

I really don't know where we went wrong, I do know my presence annoys you so I will try to stay out of your way. It upsets me the way you speak to me, it's with such anger. Are you annoyed at me for standing by Courtney when she got pregnant, what would you rather I did.... This is my last try at an olive branch but if you really do not want to get on with me then I will keep out of your way. We are a family or that's what I thought .. Here for each other as you know I would be for you but I would never say to my mum "fuck off" whatever she said to me. I know you've said I need to earn your respect but is not carrying you during pregnancy, going through a caesarean operation, spending 5weeks in intensive care with you and bringing you up to be independent enough to earn enough respect to actually be civil...

I know the feeling well ..

...also think having the baby around is causing havoc with her hormones.

She is behaving a bit prima donna'ish and I can see why you're so exasperated [and why you felt like leaving home]. But I do think she's hurting and is probably confused about her feelings. No matter what the age is or the circumstances there is always sibling rivalry; sounds like a small child doing the 'behaving badly to get attention' thing to me [but I'm no expert]. Think all of your family would get a shock if they had to live by another families rules. My son's ex girlfriend spent a lot of time at my house for several years and I often used to shut myself away in my bedroom when she was here; I'm very easy going on the outside but sometimes I just want to shout to everyone 'get out of here and leave me alone for a bit'.

I haven't rewarded her sister, I've stood by her as I would do with both my daughters, I just feel sad that the bond we had left several years ago and I don't know how to get it back.
She seems to get annoyed by my presence, I have to stop thinking of what a lovely daughter she was and how close we were up to about her 15th birthday as this just upsets me too much.
She always was a daddy's girl and our youngest always had anger issues up to 2 years ago but now says you never appreciate your mum until you are one.
I really don't know what to do, I don't want her to leave but I don't want to be spoken to like this anymore
My mum and dad died in the last few years so I really have no one to ask advice and I'm still in my 40's...

I wouldn't dare. In fact any conversation turns into an argument. She was always the good one but then her opinion of herself over inflated so she could do no wrong.
The other day she said "I washed up the pots and you never thanked me" to which I said we all pull together and muck in, no one gets thanked for each task that's normal day to day life so what makes her feel we should all thank her.. She has forgotten what thank you is as I haven't heard her use a thank you in years

Sounds like a normal 20 year old to me but could be reacting to what she sees as you rewarding her younger sister for getting pregnant.
Twenty is fairly young to be struggling on the property ladder by herself unless of course you are going to help with the purchase of a property.

This must be awful but I can see her 'spoilt' point of view. Her sister messes up and is rewarded with a family plus adoring mum! It is very easy to see your children as 'the good one' or 'the selfish one'. Both are very lucky to have such a supportive mum. Only advice I can give is to avoid discussing DD1 with her sister. It will only end in tears.