I had a family member that played favorites outright like this too and it pretty much damaged our relationship near unrepairably until I reached adulthood, moved out and didn't talk to her for three years. She realized real quick what she had done, we reconciled, and now she's a very important part of my son's life and mine.

My fiance went through a similar situation with his grandma and his twin and younger sister. To this day she constantly belittles him and brags about the other two, while still relying on us to regularly assist her whenever she needs help. The day our son was born, we sent her a picture of him since she couldn't be with us to see him born. She called immediately and told my fiance that our son doesn't look anything like him. We just hung up on her and now we spend extremely limited contact with her. We won't expose our son to her toxic nature, and neither should you and your husband.

You both brought two beautiful lives into this world and their grandmother should be proud to have them both in her life. If she can't cherish them both, then she shouldn't get either of them. If you have to compromise for the sake of your husbands sanity, please do yourself the favor and follow this advice:

1) For every day she spends alone with your daughter, she has to spend that same amount of time with your son. Similarly, for every non holiday related present she purchases for your daughter, she has to do the same for your son. At least then she still gets to spend her "mommy time" with your daughter but your son will also get some special time with her as well, making it seem more equal in nature to him.

2) When your daughter is away with gma, use this as special time you and husband can spend with your son. Maybe do a special day trip or movie and game night type thing. Likewise, when your son is away with grandma (if she even abides by the first rule) you should do the same with your daughter.