Hello, I'd like to help you with your question.
There could be two possibilities here. One, your sister had issues before the divorce and the stress
of the situation brought them out. Or two, she is reacting to the divorce in a bad way and does not have the resources to cope with her emotions so she is taking them out on everyone else.
If your sister had problems before with anger, blaming others or any similar symptoms, the divorce could be making her symptoms worse. Some people have issues with anger or personality disorders that are manageable as long as their life is not too stressful. But when something significant happens and they feel out of control,, the symptoms can worsen and the person becomes difficult to cope with.
The other option is that she simply has a reduced ability to cope with her situation. If this is new behavior for her, then the stress of what she is going through is too difficult for her to handle and she is taking her emotions out on everyone else in order to deal with her feelings. Some people do not do well with crisis or life's difficulties. They don't have the coping mechanisms so their emotions become exaggerated and they act out.
If your sister is not willing to get help, it can be very frustrating. Getting someone to get treatment when they refuse is always difficult, especially when dealing with an adult that you have no control over. You want them to see the importance of taking care of themselves but for some reason, they refuse. It can be upsetting when you care about the person and they will not listen.
If you can, try to see if your sister is willing to see her own doctor. Sometimes a person will talk to their doctor when they refuse to talk to a therapist or psychiatrist. If she is willing to see her doctor, call ahead before her appointment to let the doctor know what is going on. He or she may not be able to share anything with you, but you can certainly tell them what you are witnessing with your sister and they can address it with her.
You can also enlist the help of friends or other family members. Sometimes a person will listen to someone from outside of their immediate family because they are not so easy to dismiss.
Here are some other resources to help you with ideas on how to help your sister:
I Am Not Sick I Don't Need Help: How to Help Someone with Mental Illness Accept Treatment-Xavier Amador.
You can also try to protect yourself by minimizing contact. If she tries to see you and seems out of control, tell her you are too busy and can't talk now. And if she tries to say something cruel or hurts you in another way, say something such as "I'm sorry you feel that way" and walk away. And if you feel she has become a danger to herself or others, call your local ER for instructions on how to legally force her to have an evaluation. They will know the local laws and can guide you.
I hope this has helped you,