Am I the asshole?

My sister ( who I’m not very close with. We just got into contact like 3 years ago) she text me and asked if I could watch her 1 year old so she could go out to a concert with her boyfriend. I told her I might be going out and she said to let her know asap so she can buy the tickets or not. Well I ended up getting the flu and text her “hey I’m not going out anymore but I’m super sick “ and she said “oh that’s perfect cause he’s sick too! So it’s fine” I was annoyed with this because I don’t want to watch a toddler while I have the flu. Especially a sick toddler so I said “no I’m like super sick. I have a fever and my body hurts so bad” that was the end of the conversation she never responded so I assumed she understood that I said no and then the next day she texts me “on my way to drop him off. Can he just stay the night so I don’t have to come get him and I can have sexy time with my boyfriend?” I told her no I never agreed to watching him I’m sick and she’s mad because she said she bought the tickets already. Now I feel forced to watch him but I feel soooo awful. I have a migraine, my body hurts and I can barely stay awake