Stepmother from HELL!

I don't know what to do! I need advice!!! 
My stepmom treats my dad SO bad!! Like she will talk shit about him to my family and to hers.. In front of him!!!😠 
She will always make him pay for her things and makes him buy her and her daughter stuff! Yes, he is reasonably wealthier than her but it's not way to treat someone you 'love'.. IF she actually loves my dad at all! 
I'm engaged and she hates my fiancé.. Again I don't know why.. He's never met her before me and is so rude to him!
I don't get to see my dad much but everytime we arrange a date to meet.. (Without her.. Because she causes drama) she will tell my dad SHEs taking him out so he has to cancel on me! 
She's so horrible to me and when I first started seeing my fiancé she called him behind my back  and told him he must leave me alone and not date me.. She never gave a reason.. 
My dad calls me everyday when she's at work, and I tell him that he must leave her and she's just using him but my dad is very spiritual, and thinks that he needs to stay with her to help her become a better person.. And I tell him not to waste him time. In all of the years they've been together she's treated my dad like shit and it hurts me more than anything!! 
I snapped a week ago and told her how I felt and she didn't say anything.. And made a lie up that I cheated on my fiancé in the early days.. I don't know how she got that as my fiancé was my first everything... She's a compulsive lier! 
My wedding is soon and my dad asks if she could come to the meeting with the family to organize it and she was so rude to my fiancés mom... So now I've had it! I've banned her from our wedding and from our lives! 
I just wish my dad could get the courage to leave. 
She's moving in with him soon in his new big house and she's spending all his money on redecorating it for her and her daughter! 
Please help, what can I do!!! I cry everytime I think about my poor dad and what he's going through!!😩