My mother in law favors her first granddaughter more?

My mother in law favors her first born son's daughter over our daughter. She showers her with presents and has gone to every one of her birthday parties, but she doesn't care about our daughter at all. She refuses to come to her first birthday in May. Her step daughter is going to be having a baby soon and she cares more about the unborn baby. It seems my husband is her least favorite child. I confronted her about it and she said I love Kendra(our daughter) just as much as I love Brooke. Just like I love Kirk (my husband) as much as I love Eric (his brother) Which isn't true either because she always favored Eric. I don't know what to do. I'm ready to tell her to **** off and stay out of our lives! My husband has already tried to talk to her to no avail and I'm just plain fed up with it. What do you think I should do?