Family strugles, this isnt fair, would you have some advice?

well my parents are devorced. my mom and dad live about a half hour away from each other and i live with my mom. i see my dad everyother weekend and its hard because i miss him and my little sis. well my dad wants to take me for the summer, between school periods, and i want to go =) usually i'm with my mom all summer and babysit my little brother. well i'm growing up and i want to see my dad more and he's missing on my life as i'm growing up. i want to tell me mom i wnat to be with him this summer but she'll start crying saying i'm leaving her, she plays a guilt trip and she tells me i can't anyway cause i have to babysit my little brother all summer. i've tried suggesting i go for a month over the summer last year and this is what she did so i ended up not going. how do i bring this upon her. i really want to be with my dad more =(