@user1477391263 I think that I did read the OP's post carefully, but I think that I have interpreted it slightly differently to you (I know that your above post wasn't aimed at me, but I think that I feel pretty much the same as a lot of other pp's here).
"Of course we are trying our best but we are devastated for her and for her sister who I fear will need to be her carer when we are gone. I look at DD1 sleeping at night and cry for her future;..."
To me the OP is saying that she fears her daughter will need to be her sister's carer, not that OP fears that her DD1 will feel the need to be, but that the OP fears her DD1 (will in actuality realise that she) will indeed need to be DD2's carer. I think that what might seem to be a subtle difference, is actually a massive one.
So to me the OP is not saying
"who I fear will feel beholden to be her sister's carer - but of course I have made sure from the start that I am doing and saying everything in my power to make sure that DD1 never feels like that. I have not only read any appropriate books on the subject, I have also had a couple of sessions with an expert psychologist, on how to approach DD1's upbringing to ensure that I and my husband never put any pressure or expectations on our DD1 to be DD2's carer when we can no longer manage, or are no longer around.
We have also learned how important it is to bring DD1 up having her own interests outside of our family set-up. The psychologist also stressed that however difficult we find it to do, we must soon start letting our DD2 go away for respite care in a residential setting every so often, so that both DDs get used to the idea that this is a good thing to do."
@lydiangel83
You could say to your DD1 how important it is that DD2 learns to live and be happy in that setting, because once you get too old to care for DD2 that will become her every day life and her home. Also, please try not to have even the slightest hint of a question in your voice, or even a hint of regret, if you can help it, just try to say it in a matter of fact - it is what it is - tone of voice.
As @user1477391263 suggested, I am reasonably sure that your DD1 will still at some stage convince herself that when you can no longar cope, she will be - will want to be - DD2's carer. However, unlike @user1477391263 I do feel that your present thoughts do mean that you are deliberately (but not nastily deliberately) bringing DD1 up to be DD2's carer, I also believe that you are doing that because your mind has not thought - or cannot think - of it in any other way. I am sure that you only have great love for your two daughters, and that your thought processes are done with love, not realising that there are choices, and that there can be a better way, for all concerned.
So, depending on expert advice, if your DD1 tells you that she intends to take over DD2's care when you can no longer do it, I think that you should tell your DD1 that you are so proud of the woman she is becoming, and you love how much DD1 and DD2 love each other, but that DD1 being DD2's carer would not be in DD2's best interests, even though you know that your DD1 has DD2's best interests in her heart.
DD2 may even outlive DD1, or any other unforeseeable thing could happen in the future to change things dramatically, so it really is in the best interests of DD2, for her to have a life independent of her biological family. Once you believe this yourself (but I fear that that will take a long time for you), it will be much easier for you to convince DD1 that you are speaking the truth.
I am in awe at your capacity for love @lydiangel83 and that you want to spread it even further for what seems to you to be good reasons. But I think you have already admitted that after DD1, you might not have thought through your expectations of a second child, and I fear now that you are again not using the best commonsense in your mind this time either.
I think that you need an in depth discussion with your DP, possibly in front of a mediator or counsellor, but at least somewhere where you can listen to each other very carefully, and without shouting at each other, or becoming overly upset. Maybe you could both go with notes you have made, just to help with the clarity of your thought processes. Hopefully at the end of that - or several - discussions, you will both agree on what you should do next.