Is it normal that my boyfriend's sister treats me this way?
I've been with my boyfriend for 5 years, and I'm not very close with his sister. The problem is, I have found out that she says horrible things about me, which she makes up - like how I supposedly gave her a "dirty look". I happened to get sick once with something rather serious, and she told her friends without asking me first. Bear in mind that I have always been nice to her, always bought her really expensive gifts for her birthday (even though we aren't close - just out of respect for my boyfriend, and to maintain a good relationship with his family). I also cleaned the house she shares with my boyfriend every single day when I stayed with them for a few months, and she accused me of throwing her things away (she later found them, but never apologized to me).
I confronted her about it in a very respectful way, and asked her to simply be open with me so that we can solve whatever problem as adults. She never explained why she did those things, and just said sorry, but she keeps badmouthing me. Is this normal for a boyfriend's sister to behave this way? And is it normal for her to gossip about my health issues to strangers without my consent?
She is American and I'm not, so maybe this is a cultural thing I don't get.