The Goq,
Is there anyone who knows your family who you could ask advice from, such as a rov or rosh yeshiva? I feel that it’s not for we in the CR to advise in such a case, since the proper course of action depends very much on knowing you, your niece, your brother, and, very possibly, your sister-in-law and other family members.
It would seem safer to remain quiet about it, and certainly not to put anything into writing, unless someone wise, who is familiar with the situation, advises you to.
Good luck, and just keep in mind that your “virtual friends” are feeling for you.