Jealousy

[Amimi2]

My older daughter is jealous of my relationship with her younger sister (now in their late 40's) and I believe that the frustrations she experiences, results in rows between her and myself over just about anything. I love her dearly and now live with her, but I am finding it really hard to avoid saying anything that might be misconstrued by her as criticism of her. She is unhappy and so am I. We get on fine until we meet up with the younger sister and that is when it all goes wrong again.

If you live with your older daughter DD1 this will become the elephant in the room if left unaddressed. I think you need to approach this in a gentle probing way- find out the facts first and then explain how it makes you feel.
The next time it 'goes wrong' with your DDs make a mental note of what your DD1 said, how she said it, tone of voice etc. After the event and when you are alone gently enquire along the lines of ' DD1, can I ask why you said / did bla bla to DD2 today?' Be prepared for an outburst ' I don't know what you mean?' / ' I didn't say / do anything, it was her she always does the to wind me up'. Be persistent and rephrase with an explanation ' I'm just trying to understand, you are my DDs and I love you both very much, it hurts me when I you fight / don't get along' etc. 'I don't know if you are even aware, but you sound quite angry / sarcastic when you speak to her' etc..
Difficult conversations like this are so much more difficult when it involves those we love but it sounds like DD1 may not be feeling the love and if she is jealous of your relationship with DD2 you need to get to the bottom of why that is, then you can try to put it right. Be prepared that you might be part of the reason she behaves like she does. Have you tried to think about your own behaviours? Do you treat your DDs differently for example? Such behaviours are likely to be deep rooted over the years and will take some un-doing which will be uncomfortable for all.
Most important is that you remain calm and speak from love when you have the conversation.
Hope this helps a little.

You've got years of what has caused this to unravel, and the pressure cooker of being in the same house.
Even if she's in the "wrong" her feelings are very real.
I wouldn't even try to sort out something this complex only own - would she see a family therapist with you?
I think anyone would find it hard to listen properly, without getting defensive.