Your sister is suffering from persecutory (paranoid) delusions. A delusion is a fixed irrational thought or belief. People with delusions can’t be persuaded by facts. The believe what they believe. The content of delusions can be bizarre or non-bizarre. Bizarre delusions are just that – bizarre. For example, a person might believe that he is really from another planet. Non-bizarre delusions can sound a bit reasonable but are highly unlikely. Your sister’s idea that the government is out to get us is a good example.
Delusional thinking is often associated with schizophrenia but it can also be a manifestation of an extreme anxiety disorder or the manic phase of bipolar disorder. People with persecutory delusions are often angry, irritable, and resentful. They are very frightened people. Some people with delusions can function pretty well as long as people don’t argue with their irrational thinking. I’m struck by the fact that your sister has been married 5 times. That means that she was able to find a partner 5 times! She must have periods where she settles down some.
Your sister needs to see a psychiatrist. An anti-psychotic medication might quiet the delusions so she can live a more normal life. You won’t be able to convince her to see a doctor since she doesn’t believe that there is anything wrong with her. If she becomes violent or irrational, though, you could take her to an emergency room for evaluation.
I hope you can get her to a psychiatrist. This is hard on her and on everyone who cares about her.
I wish her well.