King Hiranyakashipu tried many ways to kill his son Prahlada. Holika was his sister with a boon that a fire can not burn/harm her.
To kill Prahlada in fire, Hiranyakashipu asked her sister's help. She suggested that she will sit in fire with Prahlada in her lap, so this way the fire will burn the Prahlada.
Now when the fire was finished, Holika was burned into fire while nothing has happened to Prahlada.
So, My question is Why Holika got burned in fire even when she was having a boon not to get burned in the fire? In other words why the boon didn't work?