Soul: the immaterial part of a person; the actuating cause of an individual life
Sister: a female person who has the same parents as another person
Soul Sister: Someone who fully understands you. Like a soulmate, but not someone you want to marry and make babies with. The sister of your soul.
1)As quoted in Confessions of a Teenage Drama Queen, "Ella Gerard you are the sister of my soul"
2)Hayley just gets me, she understand everything about me, she's my soul sister.