I need help with something?

I have an 11 year old daughter, Macy, and her dad and I got divorced when she was 3. I remarried when she was 8, and him and I had a daughter, Raye. Well, Macy Loves Raye so much, and helps me take care of her and I just had another daughter, Natalie, and Macy is suddenly acting very upset and is not showing as much compassion towards Natalie as she did towards Raye, why is that?