My sister wants to break up with her boyfriend and my mum is making it harder than it needs to. So to make a long story short, my sister (18) is going to break up with her boyfriend (19) of 4 for personal reasons. She spent the week talking to her friends and her teachers about it because she still likes the guy and doesn't want him to get to badly hurt. Now my mum thinks of this boy as her own kid and doesn't want my sister to break up with him and is trying to convince my sister of not breaking up with him because 'what if he does something to hurt himself?', she also wants my sister to talk to people in her church that supports my mums view and has
went as far as to threaten to call the police of my sister stepped out the house to bring her back. 
How does my sister go about taking to our mum abour this? This has caused so much arguments between the two.