Oh, my, how devastating. You have every right to feel angry and bewildered.
I don't want any of this post to sound trite or condescending; after living through the death of a child, I would just like to say please please please try to be gentle with yourself, and know that life will NOT always feel this way.
Take one day at a time, and if that's too much right now, just do one hour at a time.
Pull out any self-care/restorative ideas for yourself...could you get a massage, talk to a counselor, sit alone in nature, write in a journal, etc? Your mom is going to need you for many months, so it's vitally important that YOU keep filling YOUR needs, otherwise you won't have anything left to give. Share the load with your siblings, and let the doctors help decide when would be the right time and way to share the news of her partner's passing with your mom.
You don't have to do this alone. Have people been asking what they can do? Then give them something specific. "Yes, actually, if you could...(for example) cut the grass, do some laundry, give my brother a ride to the airport, find a home for this pet, come with me to the hospital, arrange to have my mail held, bring us a meal on Friday, etc etc we would be so grateful." Most people who offer really would like to help, they just don't know what to do. Delegate anything you can to someone else.
Most importantly, you're doing the best you can, and that's all you can do.
I'll be praying that you can find some measure of relief in the coming days and weeks. And God doesn't hate your mother; never, never could He. We live in an imperfect and often nonsensical world in our imperfect bodies, and sometimes things just go awfully, horribly, insanely wrong. But God is ultimately the essence of love and perfectness. He loves your mother with this perfect love and never wishes evil.
So, so sorry this is your reality right now. I pray that despite this nightmare, you can find some clarity and calm. Peace to you.