The only time I reported to anyone was when I called Children's Aid to check the situation with the children of my son and his girlfriend. They WERE in danger and taken to a safe place until several months later when their mother got back into recovery and showed some decent clean time.
Two things require intervention, I think, physical abuse and endangerment to children. More than that is interfering and perhaps getting in God's way.
Keeping your sister in my prayers.
Hugs