P., that's such a tough situation to be in, for her and for you.
While I do not have experience that compares to this, I can say that I identify with your position. It hurts so bad to watch someone you love go through something like this, especially if you feel desparately that you want to fix it.
The hardest thing, as a "fixer" is realizing that there are some things that you cannot fix. It takes a lot to set boundaries and realize your limits.
You are very aware of your sister's problems and you are a great sister to keep her in mind despite how difficult it must be.
Number one, you have to look out for you and your family first. I know you know that, but I say this because sometimes, some people are toxic and you have to know when to step back. Second, is she open to any help or changes in therapy? If she's not, then you don't want to touch that one at all because it would be a waste of your energies. If she is, then you might see if you can be a sounding board for her while she tries to find a new provider, etc. Third, think about what you absolutely could do for her immediately, today. Support her. Call her, email her or whatever and tell you that you love her every day and are thinking about her. If you feel like you are hurting her, adjust your tactics and try expressing it in a different way. But no matter what the situation is, if you can, in a way that is healthy for yourself and your family, reach out to her and let her know that she has support and that you care about her, you will be giving her the best gift you can.
I wouldn't bring up the surgery or the miscarriage-let her bring that up. But do contact her and let her know you love her and support her. Losing a baby is one of the worst things that can happen to a woman and having friends and family that are willing to listen to you and that offer love and reassurance, well, that's just a must have to get through something like that. Equally, as a mother who has lost a baby, I get really uncomfortable when people bring it up. I don't mind discussing it when it's on my mind and I will usually open that discussion up if I want to talk. But it does hurt when I am not open to it and people suddenly want to start talking about it.
P., you are a good sister. All you can do is the best you know how. Hang in there.