help...I dont know what to do? :(?

Okay...So my little sister is crying every nite because she misses our
mom and we are away at my dads for the month. And My mom says go tell
my dad and step mom BUT my sister is begging me not to! I have no idea
what to do. She does this every year, she has a phone so she calls my
mom and texting her that she is crying and she misses her. I do not
have the ability to use my phone now. But i need help! And sometimes
my dad might take her phone. But SERIOUSLY i would too! she is getting
herself all worked up! She CRIES every single dang night. And I dont
want her to be mad at me if i do tell. But my mom said i should. And
all my sister is going to do is scream and cry. We see my mom in 2
weeks to go to a concert :D But then we leave again for a week. Please
help!!!! :( I dont know what to do. I feel like im stuck in the middle