How do i control my nieces jeolousy? HELP!

im having a problem my husband 4 year old niece is getting out of control with her grandmother, ever since my mother in law started taking care of my 2 month old baby my husband niece has been getting jeoulous trowing fits and screeming that she does not love my baby and she no longer wants to go to grandmas house. Im having this issue of knowing and seeing that my mother in law sadness of having her granddaugther not wanting to be with her only because she is jeolous of my baby. It upsets me because all of this cause of jeolousy is because my mother in law passed the limit of spoiling my husband niece and now that she has a new grandson she is so happy in love with him that she plays with him and takes care of him and my husband niece is seeing all of that and does not like that  when every person that comes to the house to visit my baby at my mother in law house or my father in law goes straight to see my baby and play with him and then play with my husband niece since my mother in law also takes care of her. On thursday my mother in law took my baby with her to the dentist and her daugther gave her ride which is the mother of husband niece and she trew a fit because she wanted to be the first one to be inside the car before my baby and when her mom put my baby first in the car she trew a fit all the way to the house screeming and crying and yelling that she does not love JanCarlos my babys name..... what can i do to stop this jeolousy and what advise can i give to my mother in law?

Your mother in law needs to reassure her granddaughter that she loves her just as much, even though she has a new grandbaby. Tell her to really "emphasize" how much she loves that she has a "BIG GIRL" to help her with the baby and how wonderful it is. Your mother in law can also tell her stories about a little girl who helps her mommy and grandmother out with her new baby brother..sometimes children relate to stories better. Encourage her to spend time with the baby and help out...once she realizes that everyone loves her just as much, she will calm down about the new baby.