AITA for breastfeeding my neice?

My sister (25F) has a four month old and I (28F) have a six month old. We are very close, and she asked me to watch her baby overnight last night. She brought bottles and pumped milk, and informed me she’d never tried giving her a bottle but “it should be fine” and left. A couple hours later, her baby was hungry. I prepared a bottle and tried feeding her the bottle, but no matter what I did she wouldn’t take it. She just kept crying. After two hours of trying to feed her a bottle and then trying to spoon feed her and her screaming, and me being unable to reach my sister, I informed my sister of what I would be doing and I breastfed her baby. I guess she didn’t check her phone for several hours because I ended up feeding her baby twice before my sister responded, and she was furious. She said I had no right to do that and I should’ve figured something else out. So I’m wondering, am I the asshole here? She hasn’t spoken to me since picking my niece up.