Let down by sister

My sister offered last week to come round this evening and look after my little one so I could get a few hours rest and get some much needed admin done. She had even offered to stay overnight and do one of the night feeds. At 3pm today she messaged to say that she could not come until 9pm. I was expecting her around 6 as that was the original plan. I’ve instead suggested that she come tomorrow instead as it will be more disruptive for us to have her turn up while we are trying to settle our LO for the night. AIBU to suggest this as she says she could still help with the night feed if I wanted.