Wow, this made me soo sad. Im so sorry, and you've got to let her know somehow that its hurtful.
Yeah I really hope it will change when baby girl gets here too, I just can't understand why she would act the way she is acting, out of five siblings I'm the only one who has kids so these are her only grand babies, I just don't want my baby girl to start to question why grandma doesn't treat her the same as her brother, it will break my heart and I think that like you I will stop letting my mom see any of them unless she changes, and thank you for sharing your experience, really appreciate it :)
I have 5 daughters and my mother has always favored my oldest. It drives me nuts. She will only ask my oldest to spend the night or to go somewhere with her. I just tell her no now. My daughter agrees that it is insane and she is only 10. It breaks my heart when my other kids question why they never get asked to spend the night, etc. My mom has been wanting me to have a boy and now that I am pregnant with a boy and I can just see her completely forgetting about all of my girls. I honestly dont like my kids by her much at all anymore. I know how hard it is. Sorry you have to deal with that. Hopefully things will be different once your little girl is born :)