Do you assume when tell someone about events that they are going to come to it?

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My sister-in-law messaged me about tonight because my nephew in high school has his last choris performance for the year. I have never in the past really gone to any of his school events because he isn't my child and I don't ask when they are. I know my sister-in-law mentioned it to be but I never told her I was going. So she just assumed I'd be free when I have 3 kids ages 13-9 years old at 7 pm on a Thursday night that I would be free to go. Of course, I kindly responded just now when she asked if I needed help saying seats. No! Letting her know I have to take my youngest to his pack meeting for Boy Scouts. Which is at the same time as my nephew's choris but also if we went to his choris thing my kids would get to bed till almost 9 pm if not later and their bedtime is around 7 because they have to be up early for school. Now she is making a sound all sad that I won't be taking the kids because she wants me to get on stage and sing the school song. The high school song never had lyrics when I was there they were not written till after I graduated.