I feel bad for telling my sister no

I love my nephew, I do. But he's Breast fed and she told me she didn't ever bottle feed him, she takes him to work with her and everything. I am a daycare teacher for babies so I am used to getting a baby used to a bottle so they will eat.. But I have those babies everyday and eventually they learn that only mom gives boobs and daycare gives you a bottle but that takes like a month and a lot of phone calls to a mom telling her her baby hasn't eaten for us and that we have tried everything and those moms will start using bottles at home too and Breast feeding just so they get more used to it. 

I stayed up the entire night with a screaming baby trying over and over again to get him take the bottle and she wasn't answering her phone. She wants me to watch him like once a month... That's not enough time spent with a bottle unless she starts giving him bottle  too. Eventually he did suck the bottle but he would stop and scream at me and then drink some more because he wanted a boob point blank but he was also starving so he gave in which is AWFUL. I felt so bad. He screamed at me for 5 hours straight and it was a good bottle too.. I know this sounds selfish but I like having a baby free weekend especially since I watch 8 of them everyday. 

Just ugh. I feel really bad that I told her no. Am I being selfish???