I think some of the pps have come down on you very hard. It's an adjustment for everyone in the family when an adult child moves back home - I should know; I've two of mine back home after several years away. They would prefer to be living independently but for various reasons this is the way it has to be for now. Add to the mix a 19 year old who's at uni but hasn't lived away as yet. It's challenging!! I love them all dearly and part of me is happy to have all my chicks back in the nest but there are arguments (with me, mainly over housework!!) and they do wind each other up. It's a madhouse sometimes!
They are all strong-willed so it's inevitable that they will clash at times, but they are also incredibly close to each other.
Your DH needs to step up here (I know, I've a useless prick here too) - you must be so torn, knowing that your mum needs you and that your DD2 needs you! For those saying you should go home - don't you have a heart?!!! I lost my mum 16 years ago and I miss her every single day, but at least I have no regrets because when she was terminally ill I did all I could for her while balancing working fulltime with rearing 3 young children under the age of 9.
Maybe your DD2 should fly out to you for a weekend, just for a break from things? Might do more good than harm? And, is there any way you could bring your mum home with you?
Your eldest's behaviour isn't acceptable and she needs to know that in no uncertain terms. She probably didn't want to have to move home either. She needs to find a job, any job, to occupy her and get her out of the house. I wouldn't be skirting around her re phone calls though. I'd openly say, "I need to speak to you when X isn't around" - what harm if she does see/hear that?
How did she get on with her flatmates at uni? Did she behave in this way with them? I somehow doubt it. Explain to her that, now she's an adult, she's now living in a houseshare arrangement at home, and needs to behave accordingly. Be firm with her. I've said that to mine, but it is an uphill struggle sometimes, and you have to stick to your guns.
Hope your mum's health improves and that you can get this sorted out so that everyone can be happy. Whatever about your eldest's behaviour - and you know it can't go on - it's hard to read/hear some of the horrible comments that have been made about her. She's still your baby, at the end of the day! Take care x