Would you do this if you were a grandparent? (Read description)

Ok, so my mother in law (the grandparent mentioned) called me to ask if I could pick up my niece (my sis in law’s daughter) from elementary school because she was busy shopping for a dress (note: nonessential item). She knew she needed to pick her up at a certain time like she does 3x a week because her daughter (the mom) works & depends on that income. However, she decides it’s ok to put her shopping as more of a priority, so she calls me. I can’t do it because I am busy & also I don’t have a booster seat for my car for my niece to sit in. So anyways later I find out that my niece’s mom (her daughter & my sis in law) had to take off work to get her daughter from school because her mom (the grandparent),who had plenty of time when she called me to get back, decided shopping for a dress she wanted was more important. Now, I know for a fact that my sis in law needs the money from her job, so I personally think it is very rude that the grandma made her daughter take off work when she had promised she would pick up her granddaughter. Thoughts? As a parent or grandparent, would you ever do this? I personally feel that it was selfish & irresponsible on the grandparents part.