Please help me figure this shit out

My sister asked me to babysit her 2 kids the other day i already had a hair appointment. But i had agreed as the time worked. But my hair dresser changed the time for my appointment i am 37weeks pregnant and i really wanted it out of the way incase baby comes any time i couldt get another appointment for another week. I told her the day before as soon as i knew that i may not make it to watch the kids and im sorry. She ignored me for 3 days straight and now comes out guns blames saying im un remorseful ungreatful and selfish. I always babysit for her i drop whatever im doing to always be there for her i love my neice and nephew so much. This one time i couldt make it im suddenly the devil. She goes wait till u have ur kids and no one helps u blah blah. I think she over reacting and her insults are uncalled for i feel like she thinks i should just be her on call nanny all the time and must be available for her im not allowed to make plans or change plans. Am i in the wrong?