"Don't be scared, they are your big sisters," Cross said to very young and trembling Alina. "You sensed them and they sensed you, they been wanting to get to know you."

This takes place after week Alina being saved by her father from her cruel mother. She had week of getting comfortable around him and facility. She could sense her older sisters and knew they were very powerful. Alina being scared of them first time meeting is reason since Maple is the granddaughter of first vampire and Iris is daughter of Lucifer, that's right Iris finally has cannon mother. It also explains why Iris can use heaven fire and her animal god form is made out it too.