They choose and you remember that it is their choice, not yours. It's also important that these choices be respected by the various relatives. Substance abuse or other dysfunctional traits should be confronted, though, regardless of the unpleasantness involved. These are your grandchildren and they deserve your help for a normal life. It should not change.
Some grandparents hesitate to intrude.
Yet if children lose the relationships of their extended family and their grandparents, too, it compounds and worsens their feelings of loss, confusion, and sadness. When you can lend stability, continuity, love and support for your grandchildren, please don't hesitate to go to bat for being with them. You have a right to your grandchildren. No one can ever have too many people loving them. Stand up for their rights.