When is enough enough?

I've been in a relationship with my fiancé for 3 years now. We have pushed back the wedding several times because his ex wife is crazy. He has 2 kids with her, and she is mentally destroying them. They get smacked if they say they love me and their dad. Get put in time out if they say they miss is. They are forced to lie to social workers, I have a recording of them telling us that their mommy tells them what to say and tells them to say they hate us and that we hurt them. And if we don't, mommy spanks us.  She still refers to my fiancé as her husband. She has been ordered to take parenting classes, which she hasn't done in a year. And most recently while overlooking  the girls homework, they had to write sentences for school. (2nd grade) and the sentences were. "When I go with my daddy, I cry and miss my mom." "My mom and my sister are my only family" and "my mom is my favorite person ever" what 8 year old would willingly write that? Both kids say mommy made her do it. But when is enough enough? I'm so emotionally drained; and I love the kids like they are my own. But I've made it very clear to them that I'm not mommy, they have a mommy and I'm not taking that away from them. I don't know what to do anymore. My fiancé says to ignore it. But it's always like this.