Relative canceled their birthday dinner to reschedule: When someone cancels on you, how willing are you to reshuffle your schedule to accommodate them?

My parents and I were going to go to dinner with my sister for a milestone birthday tomorrow. My sister spent the weekend away so she cancelled dinner. My mom told me that since dinner is cancelled, we’re going to do it next Sunday.

Next Sunday I’m busy doing something with my daughter’s friends and their mom. The mom is busy every other weekend for the few weekends left in August.

I told my mom I guess my daughter and I won’t be having dinner with my sister for her birthday, because we’re busy next Sunday,with the friends and their mom. Tomorrow we’re free, next Sunday we’re not.

I’m just curious what criteria do you use to determine how willing you’d be to reshuffle your schedule around to accommodate someone who cancels on you? Do you go strictly by “first come, first served” or are you more flexible?

I’m feeling like I understand why my sister cancelled but since we’re so booked up for summer weekends, I’m not sure if I should be more willing to cancel with my daughter’s friends or just stick to our original schedule.