Well, you be there for her and be there with her.
When, you both, say this to your sister, in person.
I also wonder, how your niece's new Husband, will digest this info.
Since 'you' are the only one she has told.
I also wonder, if perhaps, your Niece's other siblings... were molested too.
It is not, uncommon.
I also, suggest to you, that no matter what you hear from your sister or whatever the drama/scenario is that unfolds... you also may 'see' a new not pleasing side, to your Sister. And/or of your Brother in Law.
So, be prepared for that.
But then, who... will you support?
Your sister or your Niece... who was molested???
Because... a battle may, ensue. And unfold. As this news, breaks loose in your family.
I really feel, for your niece.
It is a nightmare to have to have lived with. ALL this time. And she even tried to protect her Mom.
Her's... was being raped/molested, over the course, of YEARS. Not only once. It was, between the ages.. of 10-14.
That is FOUR years, of being molested.
I was raped once, by a person I knew.
THAT was hell.
Can you imagine, what your Niece, went through??? ALL those years?
Again, as this unfolds, in your family... be prepared to stick up... for whatever is right. And for your Niece.
The 'ugly' side to people... will come out, at times like this.
Or maybe not.
Typically, they will say that you or your Niece are lying.
Or they will claim ignorance.
Be prepared... for knowing where you stand... and for her.