Immediately report everyone to the police.
my sister has been being sexually assaulted by her friend for the past few years, now my sister is doing it to other kids. my mom doesn't care and doesn't want to hear about it. What can help my sister?
Tell someone else ... Someone you already know, like a grandparent aunt or uncle, older brothers/sisters, or for that matter any other adult that may be able to help to stop this, .. Or call the police yourself even if you report it annonymously. It will be investigated and stopped (if it in fact is really happening) .. Your sister and the other children should not pay consequences for your own mothers refusal to accept the truth about what is happening. You need to insist that someone get involved in this .. And ASAP!
If you are not part of the solution .. You are part of the problem.
You keep asking if there is a "lesser authority" you can tell.
Do you want to end this crime, or not?
Or are you more concerned with your sister than her victims?
ANY authority you tell (other than family) is legally obligated to tell the police.
Realize that despite being your sister, she is not in the right state of mind because of her experiences. You really should report it if it's serious, because it's just starting to create a chain. It happened to her, and she's taking it out on other children, so the same thing will happen to them when they're older. I'm not trying to shame you by any means, but you are basically watching her ruin people's lives because of something bad that happened to her