unreliable co workers

I work with about 6 other girls. At least 3 of them CONSTANTLY ask me to cover their shifts, and two of them are managers!! I usually do, but it's getting annoying. Especially since there were a few times I actually needed a shift covered, (death in the family) and I was accused of lying to get out of work. BY A MANAGER. There was another time I took a managers shift for her, and there was a (bad) forest fire very close to my house. The elementary school not even 5 minutes away from me was already being evacuated and closed for several days. So I texted her and asked her to take her shift back because I was going to have to go to a Red Cross all night. She says "It's your shift now, dear. I have plans." (It's for a dinner party) 🙄