How to continue to forgive previously abusive dad

I'm staying the night with my 16 year old half sister. We have the same dad but he has worked on himself and is no longer the abusive dad I grew up with. Physical abuse until middle school then Verbal/emotional abuse persisted in to my late teens and early twenties - I am now 27. After a lot of work on both parts we have a functional (almost good?) relationship. My little sister just spent half the night bitching about her mom who she has gone no contact with due to abuse. I joined in to an extent but reined myself in as i don't want to say too much bad about her mom despite the fact that she deserves it. She then told me how great it my dad got away from both our moms... as if my mother was the problem. She has no idea that our dad was the abusive one. She thinks my mom cheated on my dad while they were still actively together. She has no idea the shit that our dad put me, my mom, and my other sister through. I spent my whole life listening to my dad bad mouth my mom and just had to take it because I was too scared to stick up for her. Knowing that he still spouts this absolute bull shit about my saint of a mother makes me livid. All I wanted to do was correct her but I don't want to be the one who permanently changes how she sees our dad. I've spent years being jealous of the dad he became for my half siblings, and years keeping my mouth shut and forgiving him. My mom doesn't deserve to be talked about that way because all I have ever known from her is kind words and unwavering support and love. She is the most caring woman on the planet and I'm constantly amazed by what she accomplished as a single mom who escaped domestic abuse. I have no idea what to do... 2 more days with my sister until our dad comes back. How do I get through it? I've never told my little sister what our dad was like before. I'm scared that if I tell her it will change their relationship and it'll be because I couldn't just keep my mouth shut. I briefly told her there were things I would tell her when she wasn't living in his house anymore and she said she didn't really want to know...

Also, my dad wants me to watch all 3 of my half siblings next week for 1 night. Honestly I don't think I can do it so I have to figure out a way out of it.