as**ole sister In-law

Im not really sure where to post this, and its going to be lengthy but i wanted some input from you ladies (and maybe men). 
Im SOO disgusted in my sister in law. I'll give a little background information so you understand. Jessica is my fiancés sister. I believe shes 27 now? So she got into drugs when she was younger (heroin) before i met my fiance and at the time ended up having a baby. My fiance gave her a place to stay in his apartment and he was only 16 at the time. She cared more about being gone all hours and for days on end doing drugs than taking care of her infant daughter. So the responsibility fell on my fiance. He was taking care of her kid for months until the oldest sister decided to take her and basically raised her her entire life. Shes now 7 years old. Fast forward to a couple years ago jess "cleans up" and has another baby. And another. AND ANOTHER. after everyone yelling at her to stop having them since she has no means to take care of them, never has food in the house, a single mattress a crib and a couch, doesn't have a car, doesnt have a job...thank the damn lord she got her tubes tied. 
We assume over the last year or so she was getting back into drugs, she started calling people for rides constantly or to watch all the kids all day and then one day she calls my fiance and says "oh well idk what to do Keith (her boyfriend and father of at least 2 babies) left because i was cheating and the neighbor told him" 
Like, tough shit? And that's basically what he told her. The kids out working 3 jobs to support you and the babies and you're cheating on him. Good job. 
Now over the last 3 months she has been calling my fiance and texting him harassing him because he wont come watch the kids for her. Last i knew they were your kids?? He works 80 hours a week when do you think he has time to come watch your 3 infants for you?
DCF was already involved because she was having problems with keith and she didnt want to follow their rules in order to get help. They would have found her an apartment (since she hadn't been paying rent) and food stamps ect. She would have been taken care of completely but she cant even follow simple instructions.
So 2 nights ago she tried calling at 12:30 in the morning and obviously we were sleeping.  
I guess since no one wanted to help since she would harass everyone to help her and then call you a piece of shit for saying you're working, (mind you she also literally attacked the oldest sister for telling her she needed help and she would be willing to drive her to the hospital)
 she went and dropped off all 3 kids at the fire station. YUP. Thats now 4 kids she's given up on.
My fiances father who is 50 years old, took one baby, the oldest sister took the other, and Keith took the oldest. Im so absolutely disgusted i would never speak to her a day in my life after this. I can't do anything but reassure my fiance it's not his fault. I feel awful because he feels like shit for the babies like somehow it's his responsibility. I can't even put into words the disappointment. And then she has the balls to scream at everyone and call them names when they're now taking care of her kids so they dont end up in foster care. 
How would any of you deal with this? I don't even know how else to comfort him, he's such a sensitive guy and i know he feels awful and its not even his fault. 😞