Dealing with over-protective people.

This happened to my sister a couple of weeks ago.

She was at a party with a mate of mine (I was ill at the time). While at the party my sister and my mate decided to go round to another mates house.
The girlfriend of my mate didn't take to this too well, and starting sending nasty emails to my sister. My mate and sister have been friends for 15 or so years, and I know that nothing would ever happen between them, but that didn't stop the girlfriend from sending the emails.

Since this incident my sister and my mates girlfriend have spoken on messenger, but have yet to say anything face-to-face.
We were round my mates again at the weekend, and nothing was said between my sister or the girlfriend. We also went out later that night onto the prom at about 1am and didn't get back until 4am. My mates girlfriend didn't go with us, and said nothing to either to me or my sister.
The next morning, the girlfriend didn't come out of the bedroom until me and my sister left.

Jam it back in, in the dark.