Assuming the veracity of all of your statements, I think your brother is a dry drunk who should've been put in jail a long time ago. Your family seems to be in pretty deep denial about just about everything. He really should be a felon.
I also think your over involvement with your niece and her affairs is misplaced. Your niece is a grown woman. Where was your involvement when she was a child being molested? So let your niece fight her own battles unless she asks for your opinion or your assistance. It seems that her choice is to also be in denial. I'd hate to see her therapy bill in a few years.
But getting to you
- if your sister is actively trying to "bring you into the fold" such as it were, you can always tell her no. And don't be afraid to hit the nuclear option if you have to, e. g. threatening to cut off all future contact if she persists in trying to convert you. Tell her that you can respect her faith from afar but you do not need to be a part of it. As a bonus, you can respect her faith but still vehemently disagree with the issues with the niece and the niece's daughter.
Do you believe the niece's daughter is in any sort of danger? Pedophilia is very, very real and I've got my doubts that it'd be cured by a little water and praying. Even pedos who have an intense wish to change and a great deal of motivation can often still be tempted (this is a big part of why there is a sex offender registry in the United States - you don't say where you are - the registry exists because, even after doing hard jail time, there are men who are still tempted. The need to protect children in such an instance is paramount
). If there is a true danger to the child, then feel free to step back in, but for the sake of protecting the child. Call Social Services if you must. You would not be the first person in the history of the world to report a family member.