My sister is gradually breaking up the family?

Lets just say my sister is the gullible type. When she was 15 shes (18 now ) she went out with a boy behind my parents back who was 21. When they found out a lot of problems were caused. Then she started dating another boy ( she was 17, last year ) she was going to move to spain and she told him she'd wait 2 years for him. Then after 2 months of living there she started dating another boy. She was saying to mum how in love she was and how she wanted to marry him. He was a dangerous guy so my mum took her back to england. 3 months after being back she started seeing the 21 year old from before. She tells them everything about our family, and complete disobeys us. She turned my friend against me and told lies, she did that with my family in spain too. So now my friend doesnt talk to me and she completely ignores me and my family. My sister hates me for some reason i have no idea why, this is really hurting my mum. How we stop her from affecting us?