My boss sent me a text asking if I would cover a shift for a sick co-worker by adding on 3 extra hours of my already 8 hr shift. I told her no because our line of work hires substitutes for these types of situations. My boss was irritated that I wouldn't cover the shift and told me she would take care of it. I found out that my boss did not request a sub, but instead had the sick co-worker come in. Now the sick co-worker is mad at me because she had to work and I wouldn't cover the end of her 3 hr shift. 

AITA for not covering her shift?