I can't say I really "forgave" my abuser, but my hatred for her did lessen as I got older. And it was around the time I was old enough to realize (since my abuser was also a child at the time) that it was very likely she was abused the same way by someone else, and someone probably far scarier than she was to me, that I began to let go of that hate.
I'm not gonna speak for every survivor, but I will say for me personally, letting go of that hate was what finalized my coming to terms with the event and the beginning of moving on with my life. It was a good thing for me personally, development wise. If I choose to view that hug at the end as more of a symbolic thing, it makes sense to me. Because the twin had a lot of that built up rage from her traumatic past, and if I spin that hug in my own mind to be representative of her letting go of that rage rather than "forgiving" the woman, that can make sense.
Though it obviously wouldnt happen at that moment lol. It took me over 10 years to get to that point. It does come across as just silly to have her hug this woman who killed her only living family member 3 seconds ago. I'm very positive its impossible to just forgive or let go of a pain like that THAT quickly.
And as a disclaimer, I just wanna say this is all just my personal account. Everyone suffers from abuse differently, and I'm not trying to say I'm better than anyone else for letting go of that hate. Hating your abuser is very valid, and you can hate them for the rest of your life and still have the same progress I did in terms of moving on and processing the abuse. Nobody ever has to forgive or let go in order to move on, but some people will. I just wanted to put this out there because I saw myself in the twin when she was raging against the world for what happened to her. I went through those same feelings, and dropping that hatred was what made me able to finally put it all behind me.