My heart goes out to you.
I read you as very hurt and very angry. I don't read you with a lot of the horrid chinks that are being laid onto you.
You may well have some deficits in emotional IQ but not necessarily.
It sounds clearly like you are not interested in contributing to the delinquency of an adult before God. Don't blame you for that, at all.
Sounds like the frayed, inadequate relationships go a long way back, though. The time to really influence her about shacking up is long, long past and needed to have effectively started at about age 6 months or so in terms of bonding, healthy affection and leading daily along the way in examples of Godly values as well as discussing such daily.
Given lack of an effective time machine to fix the past . . . I'd sure want the mind of God on the matter. Seems reasonable to me to stop the support. AT this point, sounds like you will likely need an atny to do that and I'd pick one wisely. Short of that, I guess I'd want to argue my case effectively without the anger and I'm not sure you can do that.
We know that in our end times era, children will be acting worse and worse in favor of satanic values. I hate the concept of it . . . the opposite of how I like to live. But, I think, many will have to cut their losses; turn their backs on the past and the destructive folks in their lives and go on with God. I probably would continue to pray unless and until God said to stop as it was futile. If there was any hope, I'd occasionally make overtures. But it sounds like it's well past and not worth much such.
What a wrenching experience. Prayers for your way through it. God's best growth and comfort to you,