I have something that I think will be useful. I feel a little similar if you just replace sister with almost ex-husband. I told my T about my pity for my H. His reply was "Compassion is essential here." So I look up the difference between compassion and pity. This is what I found,
http://www.sriprembaba.org/en/differ...ssion-and-pity

" When you feel pity, you don’t really see the other person nor do you respect them. You are not interested in the other person; you just want to be relieved of your own problem because the other person’s suffering is making you uncomfortable.”

" If you love the beggars on the street, put yourself in their place and you will see that giving them money won’t help them. In some cases it might indeed help, but in most cases you are only participating in the habit of begging and the illusion of impotence, amongst other fantasies that keep people in these conditions.
If love is overflowing from your heart and if you really care about the other person, find a way for them to stand on their own two feet. Compassion is selfless.”