Yes, it does seem that you are being take advantage off. The question is, do you have any idea why she is acting this way?
You say She has alienated not just us but the rest of her close family lately I think that is the nub of the question. Why is she doing this. Is it to hide relationship problems, a sign of mental distress or other reason?
I think the answer to this question should probably dictate how you respond. If it is clear that there are relationship or mental health problems, you probably need to hang on in, in case you need to be able to see signs that she is on the point of imploding or collapsing.
Have you discussed the problem with other members of your family. Perhaps a coordinated response would be in the best interst sof all.