Very worried about Niece, Wrong sorts of boys gangs etc.?

Hi guys, I started looking for work in the South East London area as I wanted to move closer to my sister and my other family around there. After finding a job in Brixton, i have moved into a lovely house in Sydenham. However my sister who lives in Camberwell, has a daughter who I get on well with, after a recent conversation with my niece I found out she likes Black boys, not just any black boys NO! She likes the ones in Gangs, not even minor gangs but were talking Stabbing, shooting, drug dealing type gangs. She's currently seeing a boy who has links to the Gang: 'Peckham Boys.' Apparently they are quite notorious, my sister is worried sick for her daughters safety but when she confronted her, my niece simply said that she doesnt find white boys attractive, just the extreme gangsters. Im worried my niece will put a foot out of place, or something will go wrong and she'll end up hurt or dead? Please help, she wont listen to anyone.