You’re not responsible for her poor decisions. She is your blood kin, though.
Convince her to give you or someone responsible legal custody of the kids. Encourage her to rid herself of the addictions, and to seek treatment for what sounds very much like manic depression.
Do this whenever it seems necessary. But, she’s going to do what she’s going to do. It’s her decision. It’s hard to still care while letting the weight of responsibility go, but that’s the fine line you need to ride, imho.
Forgive her for her weaknesses and inconsideration and sin. Pray for her and ask God to help her out of this, and to help you handle it in His way, the best way. Try to limit the damage to her children.
Beyond this, it’s not in your control.
The first thing to do is to forgive yourself for not being able to help her. Only she can do that.