She may be off-balanced but I don't think she is crazy. Her living situation has driven her nuts, but probably not literally. Her family is eccentric, to say the least. Her sister has had a relationship that doesn't work out with a "Mr. Whitaker." Sister thinks Stella Rondo is selfish and doesn't appreciate what's been done for her. In a very childish way, she takes up residence in the post office, where she works, much like a child stomping off to their playhouse and announcing that it is their new home.
Her actions might be somewhat justifiable. Her family is hard to deal with and their own actions don't warrant mature responses. Here is a bit of what Sister is dealing with:
"What he'd really done, he'd drunk another bottle of that prescription. He does it every single Fourth of July as sure as shooting, and it's horribly expensive. Then he falls over in the hammock and snores. So he insisted on zigzagging right on out to the hammock, looking like a half-wit.
Papa-Daddy woke up with this horrible yell and right there without moving an inch he tried to turn Uncle Rondo against me. ...he didn't see how in the world I ever got the mail put up at the P.O., much less read it all, and he said if Uncle Rondo could only fathom the lengths he had gone to to get me that job!"
I think Sister might have some mental illness. Remember she mentioned, "the doctor has always told me I was the most sensitive person he had ever seen in his whole life," that implies she has to see a doctor for some mental problem. As to the terrible treatment by her family, also remember that it's all of Sister's telling, thus not that reliable.