Changes in custody and Defamation of character

[Kensgirl]

I'm engaged to a man with 2 small children (3 & 5). He split from his ex two years ago. From the time the 3 year old was born, they were sleeping in separate parts of the house until he turned 1 and at that time he left his wife. His ex is a school teacher and has always said that nobody would ever believe him over her because of that. he is a great father and the children love being with him. When the mother comes to pick them up, they often cry and say that they don't want to leave with her and that she is mean to them and yells alot. He states that when he lived with her, it was often him taking care of the kids because she would lose her temper too easy with them. When he left, she demanded $1200 a month if he wanted to see them 3 days a week. He agreed because he wanted to see his children. A year afterwards - the support was revisited and lowered to $560 amonth. She said that she would take custody away if she didn't get more money and that he was being a "bastard" (in front of the children). Well now, we have decided that he will be moving in with me at the end of feb. He told his ex that he was moving in but not that we are getting married in June. She said that she has a moral issue with us living together and again wants to take custody away because of this but he could still have the kids on the weekend. The day after he tells her this, she is now saying that i told the children to call me mommy and that i told her children that she is mean. I have never stated anything of this nature. I am a mother as well and treat her no differently than i would want my ex's friends to treat me. Her latest is that she is going around telling people that i'm a husband stealer and that is the only reason their marriage broke up. She also stated that she will make sure that the children know it was all their fathers fault. I want to know if i haev any rights for Defamation of character and if she can really take the children away from him because we will be living together 4 1/2 months before our wedding. (not to mention, they lived together for a year before they were married and they had a civil ceremony and she does not go to church) Any advice would be appreciated.