Even if the reality is that SS will do nothing, doesn't mean you as her aunt have to do nothing.
You may not be able to stop a 12 year old from having sex completely, but you can certainly limit their opportunities pretty drastically. I was fooling around pretty early, but my parents put down strict rules so I had limited opportunities. They didn't even know I was fooling around, and if they had you can bet they would be all over me like a fly on shit. They certainly wouldn't have been doing me any favors by telling me to run wild. I don't regret fooling around so young, but if it were my daughter, I'd crack down hard.
This mother isn't communicating, she's treating her 12 year old child like a friend and simply not giving a shit. You don't give a 12 year old condoms, you give them rules and boundaries, and yes, if you have to, you switch your working hours to their schooling hours if you can so you can watch them like a hawk.
Really though, if she's having full on sex at 12, there's deeper issues going on. Happy children don't have sex at 12.
I don't know how close you are to your niece, but you can be a positive influence to her. Maybe her mother will let you take her after school or something. Really, there's not any other solution than discipline, boundaries, and watching your niece.