I'm going to refuse to watch my sisters dog while she and my mom go on vacation without me
But why? Why don't you want to watch your sister's dog?
I want to save on dog food and I don't like the company
It's mean to not help watch a sibling's pet when they need you to. They might not be able to find anyone else and really needs your help.
That's just not my problem.
It might not be your problem, but it's still rude to not help your sibling out. You should try to put yourself in her shoes.