Need opinions

I'm going to be a first time mom this year. And I have 2 nieces age 4 and age 1. don't like how my sister sometimes does things.. like everyday Monday thru Friday she goes to the gym and tells my mom to take care of them..my sister doesn't work. My mom works and as soon as she gets out. My sister calls her to drop off my nieces for her to go to the gym...yesterday I was at my mom's house , and my nieces were there...my mom tells me can you please take the girls to eat , I was like , why they didn't eat?? My sister just comes and drops them off , not caring if they ate or not and just goes to the gym...like seriously I got so mad.. she at least could feed them before going to the gym...and I was just so mad. Of course I took them to get something to eat, they are my nieces and wouldn't leave them to starve.... anyways I feel like my sister get soo much free time, she always leaves my nieces to my mom.. and when I have my daughter I'm going to take care of her, and not put all the reponsibulity to my mom....