Am I the asshole here?

Chelsea • Mom of 2 💜 💜
My cousin has pulled the I'm gonna leave him for good this time' one too many times. I don't talk about it unless she brings it up. Not my issue. He's definitely not father of the year to their 2 kids-only when he wants to spite her. Well, she recently went back to him again. Things seem ok for now (as they always do in the beginning). Anyway...she has asked me to watch her son tomorrow while she works. My bf & I both work full time & Saturday is our only day off together to actually spend time with each other, catch up on housework, start planning for our baby on the way...whatever we please! Plus we go to bed early, even on Saturday night, because we both work early on Sunday as well. My point is-I know he doesn't want to watch his own children, he probably wants to go hunting. So she gets snippy with me because we had plans already. So my question is..why am I made to feel like the asshole here? Their damn father should watch them.