Feminists

second the recommendation of Bancroft's books - they are essential. Al-Anon is also a lifesaver for relatives.

Was your niece abused in any way while growing up? That's one reason some women choose awful men -- they learned early. You may want to offer to pay for counseling, if you haven't tried that already.

What does your niece do for a living? If she is doing low-pay work like retail, can you offer to pay for a 2-yr degree in a health care field for her, so she can have something else to aim for besides another useless guy? Of course, this all depends on your ability to help, her ability at schoolwork, and your willingness to really put your hands in. You know your niece the best -- I'm just throwing out ideas that may work, depending upon her.