You have an incredible amount going on in your life and I think you're trying to take your family's problems onto yourself and maybe worry more than you need to. Please do talk to your Dr about this. Your biggest worry sounds like your sister. It sounds like she'll be fine. Of course you'll worry about her, but it sounds like you're worrying about her life being at stake when you said she'll likely be cured.
I worry about my elderly parents sometimes, too. It sounds like there are so many people who prey on the elderly. It's scary. All you can do is tell them your concerns and let them make their own decisions though. They might see more than you think they do about the situation with your relative.
I have a 73-year old friend with Parkinson's who did two stints in the Peace Corps after she retired. She recently got back from a cross-country road-trip by herself. She's always either just getting home from some long trip or getting ready to take off again. Who's to say what is "too much" for someone?