Bless you for doing that hard, but so necessary act.
You say that your sister can't see anything wrong with her handing over the child to her bf, who really spanked her.
Ok, and if sis has an accident in her pants when she is off her head, is it ok to get some man to belt her on her bum?
To me sister sounds like a very irresponsible, brain addled woman, and not one who should be left in control of a barbie doll, let alone her small daughter.
You and your family are in my prayers.
God bless.