Ah, see, I’d forgotten to explicitly mention the stance of the twin, you’ve done what I hoped… 
Grieving
Maybe she/you can relate to it being a type of grief or she/you can reframe it as that and that can help her get out of it.
Part of grieving may mean looking at other expressions of grief like denial, anger, resentment, in this case pent up, causing the ‘hard to live with’. Guilt & pain may be a major one - those might be quickly discernible, minor ones might be disbelief & shock, or bargaining - those might be there, but she might be hiding them, even from herself.
And - if it is that - she may need to find ways of expressing her grief - like collecting things like leaves and pieces of wood, “calling” them things that are gone, she needs to say goodbye to, and letting them float down a river. Not out loud, that may be very private, intimate (like regrets or infatuations that we wouldn’t let even close people want to know about, because they are embarrassing or that would make it worse), or vague and impossible to put into words.
This is however only an offer that can be made very carefully, so it doesn’t become artificial, invasive, pushy.
There may be a right time for this, yet the time / realization may not be here yet.
Maybe reading/telling a story like this can help find a similar or more appropriate way of expression.
But it may also be completely wrong and a waste of time, making her feel she is not being taken seriously, because “if it were as easy as that, she’d long done it”.
Other ways might be to write a journal, a letter, other artistic expression, do something that makes you feel close to what you’ve lost, or additionally talking to someone close or professional, meditating, mindfulness - which inspires me to the physical aspect as well as responsibility of starting to self-care, like starting a short exercise routine - at the very end of a development like this: remember your laughing yoga post? Or detailing the mindfulness idea: Going out for a walk or something half an hour a day, looking at everything we see and calling out its name (a technique from happiness training, similar to a happiness diary, cf. in my blog entries “Reasons to be cheerfult” and - much harder - “Things I love about myself”). Praps animals or kids can be a part of this, like “borrowed”, not as an additional burden and problem… or just watching them play, in their happy carelessness, stroking them, giving them something. But again for someone sad this can have the opposite effect of making sadder, as we see the big difference and so distance to ourselves - again the time & situation may be vital.
Not sacrificing ourselves
If the people around her stop or don’t start sacrificing themselves for her, that may help her end her feelings of guilt, wanting to sacrifice etc. and instead help her start taking responsibility for her own life again, the same way as those around her are doing. They are then being a role model, plus taking away a possible additional burden, so making her feel less like a burden too, as well as protecting themselves…
(Some ideas for that situation as well as other situations, btw, which may “not be unrelated” to fibro… )
Reminds me (it being Sunday today helped ) - to support her looking for her own old resources, which she enjoys, what she still does do well, but also for new ones…