AITA for not spending time with my little sister?

I (23f) have been living on and off with my mom, her husband, and my little sister (5) in my hometown since the pandemic started.

I rent an apartment in a city located 40 min away from where they live (where I go to college), but I work in my hometown, and since the University shut down, there was no need for me to stay at my apartment, so I moved in with them to be able to work more, as it was something my mom encouraged me to do. I still rent the apartment in case the University opens up again. Just sharing this so you know a bit about the living situation.

I spend a good amount of time with my sister everyday, but I don’t always have the time because of my studies and work, so on my days off I try to give her my full attention.
Today is the last day before the start of the new semester and so I have some reading to do and general preparation. I have been reviewing the new material for a couple of days already, but I still have a few hours left of reading and watching videos.

When I was eating breakfast today, my mom and her husband said to my sister that “[my name] will be taking you for a long walk today since the weather is so nice”. I said I didn’t really have time to do that to which they scolded me for seeming uninterested in spending time with her, in front of her. I said that they should have asked me beforehand instead of announcing it in front of her, then.

I said I could do it later in the day, once I was done studying, but they just ended the convo by saying “whatever, then, thanks for your help” and proceeded to take her on a walk without me. I do feel like an asshole to my sister because I don’t want her to think that spending time with her is somehow a hassle to me, but I don’t know if I’m really the AH for refusing when they announced my plans without talking to me first. AITA?