Welcome back TM
I don;t have any experience with UK Child Services but others here will.
It makes perfect sense to me you and your niece are scared but it also makes perfect sense to me that the very best thing you can do for yourself and for here is continue to demonstrate a willingness to get sober and stay that way. The very worst thing you could do is to continue to drink.
You can start showing that willingness today by not drinking, and maybe exploring other face to face support options like AA or SMART or Lifering?
D