Welcome to this forum, my partner is a CG currently in recovery. I guess there is no easy answer to the questions that you ask, the fact that you are reading and learning will help you to come to what will be your own answers in the end. I think that until your sister admits that she has a problem then it will be very difficult if not impossible to get your point accross or offer any guidance or support. This is and will continue to be a source of frustration to you and everyone else involved – except her of course as she is in denial of her problem.
I think there could be some merits in explaining your concerns to her friends because they will then be less likely to provide enablement to her and help her to sustain her addiction. I know that I certainly told my own friends after they were bitten once or twice and do make people aware of possible risks. My partner gets this entirely in recovery but whilst completely in the grips of the addiction would have seen it as interfering, being controling etc etc.
It is a hidden addiction certainly in comparison to others and to keep it so will change nothing, hiding it further only enables it to carry on. There is no shame in this addiction, your sister hasn’t asked for it and will not want it but it is something that she could choose to live in control of.
Who knows when someone makes the decision to change their life it is probably very different for everyone but what I seemed to eventually understand is that they have to reach this decision for themselves and to get to that they seem to have to be totally sick of the consequences to their own actions. For that reason consider stepping back a little even if this means she has to fall flat on her face time and time again, let her pick herself up and suffer her own consequences. It is hard to do, but your energies are best focused on looking after you and providing the support to your niece. If the time comes when your sister wants to change then you can probably offer her a bit more practical support but in the meantime if you have pointed her in the direction where she could get help then there is little else you can do. When this time comes you will need your strength in bucket loads. I
Although you are worrying now it may be an advantage that you do not live under the same roof as this addiction, take it from me having the space away from it is helpful, living with it is worse than a nightmare, I think its important for your niece to get support for her.
I’m not sure if any of this has been helpful to you but what I would like you to know is that you are not alone, keep reading and sharing your experiences.
You can love your sister without loving the addiction
 We see things not as they are, but through how we are today x