so not fair

My sister and her three kiddos came down for a visit. I love to see them but it always seems to upset me before they leave. My mother has a total of 8 grandchildren but you would think she only had 3 with the way she treats my sister's children differently. I saw my mother last thursady and she said that she wanted us all to get together and go to the water park Monday or Tuesday. I called my grandmother Monday afternoon to wish her a happy birthday. My cousin's little girl had been staying with my grandmother and I asked if she was still there and that's when she told me that she had gone to the water park with my mom and my sister, her friend and my sister's 3 boys!!! I was pissed! She does this crap all the time. I could tell you hundreds of stories were my children have been treated indifferent by my mother. I'm so tired of it. I feel like my hands are tied because she's my mom and I need her in my life but I shouldn't allow her to treat my kids differently. If I mention it, she'll shame me for begrudging my nephews fun when my kids have it so much better than my sister's kids. Are my kids to be punished because I'm a better parent? I just wanted to vent. Thanks!