In Cleveland a month or so ago, two mothers were also convicted of allowing a 19 year old to have sex with a 12 year old. Not only was the 19 year old convicted of a crime and a registered sex offender but so are the mothers of both of them. What your sister is doing is illegal. If she wants to lose custody and spend time in prison and be a registered sex offender tell her to keep it up.
Parents should remember 3 things: Love your kids more than you hate your ex; when you have children the relationship with the other parent is until death; your children determine what type of nursing home you end up in.
Nothing stated by me should be taken as giving you legal advice or forming an attorney/client relationship.
Attorney-GAL in Ohio.
I've removed the knife from my back, polished it, and will one day return it -- long after you think I have forgotten.