I can't stand my SO's Mom

K • ✨
My SO is one of six kids, his mom had his oldest brother when she was sixteen (no judgement there) but continued to have more and more kids she couldn't afford, and my 25 was caring for five kids, living off the food bank and in debt about 100k because of bad financial choices. But that isn't where my problems lies, she slept with a married man and got pregnant, letting him lie to his wife for four years after their child's birth and not letting her know that this child even existed. 
Now that baby is eight, with a physical deformity that she doesn't let get her down (it makes me so proud to see her overcome simple tasks that are incredibly hard without a right arm) but her mother just drinks and cries that she's all alone because her SO left her a year ago (he was abusive anyways) and won't make the right choices for her daughter so she can continue to stay in the same place in case that man comes back. 
We've tried interventions, regular talks, asking her to see a therapist but it never works. She always says she can handle it herself. 
It makes me so upset because that eight year old will one day start to notice the drinking, be affected by it the way my SO and his siblings were, and it'll possibly hinder or hurt this child's life. She's going to need extra help, extra self confidence once she gets to those teen years, and i wish her mother could grow up and provide that. 
I also fear her mother will end up getting so bad the child has to be taken by SS, which is traumatic in its own right since my SO and his siblings all don't have the financial means to care for a child. 
I know I may seem harsh, and I have has conversations with my SO about it, but I would never tell him just how strongly I feel. Sorry for the hardcore venting.