The Unborn (2009) on Netflix

Plot summary

Casey Beldon sees strange dogs around her neighborhood and a child with bright blue eyes who follows her. While babysitting Matty, the boy next door, she finds him holding his baby brother up to a mirror; he attacks her and breaks the glass on her head, saying Jumby wants to be born now. Her friend Romy tells her a superstition about newborns and mirrors. Casey's eyes change color, and a doctor asks whether she is a twin. Her father admits that a twin brother of hers died before birth, the one the family called Jumby.