I received a letter from centrelink. It had my address and I could tell from the envelope that it was a cl letter but didn't realise it was intended for my neighbour until I started reading it and had no idea what it was going on about. I tried to return it to her personally but she was on the phone when I went over so left it on her door step.
Honestly, unless you saw her steal the mail from your mail box then I don't think you have many rights. Yeah she read it but the postie made the mistake and all it takes is a moment of lost concentration on her behalf prior to opening the letter.
I know you said you don't get on but that doesn't mean she did it intentionally.