What to do when I feel like my mother favors my sister and her children?

I am 29 years of age and have two children; I live an hour from my mother's home; my sister is 28 years of age and has two children of similiar ages in the same town as my mother. I have tried desperately to be apart of my mom/sister's lives but constantly feel like my mother and sister are tons closer. Also I see this with my children's relationship with my mother, she doesn't spend as much time with my children, whereas she spends 4-5 days with my sister's kids, when my children are around her the same time my nieces are around, she tends to share more of her attention with my nieces. She has never offered to watch my children; in which I have had to ask, whereas she has watched my sister's girls on several occions and willing to offer. I am very sensitive to my mother's feelings, but feel like I need to talk to her about how I feel, before I grow more resentful of her lack of interest in my live and my children. What do I say? And would an email be too detached?