To get this achievement, you need to finish the game having saved both Rebecca and Jill.
To have Rebecca with you during the final boss in the lab, be sure to save her when she is about to be attacked by a Hunter upstairs in the Mansion (there will be a cutscene showing her in danger). Be sure to head there immediately to kill the Hunter, else she will die.
To get Jill, , you will need to collect 3 MO disks. One is in the Mansion, in the Tiger statue room, the other 2 are in the labs. Once you have them, head to the door closed by 3 locks in the labs, put them in and free Jill from the prison.
Then just finish the game as normal and this should pop during the final cutscene.