OP here. To clear some things up, my dad and mum divorced like 9 years ago. The fact that he has a girlfriend doesn't bother me at all. It is more the fact that I know this trough my brother that bothers me. I'm at my dorm for a month because I have to take exams (in Belgium it is normal to spend the weekends at home and the weekdays in your dorm, but now I am at my dorm for a month), so I haven't seen my father for quite some time. When I found out she is going to move in with us was a complete shock for me. I don't know her, nor does she now me, and my father hadn't even told me about her. He called me (after I called my grandma because I was very upset) and said not to worry. If she's moving in, it will be for November. By that time I will have met her (hopefully).