It's because internal "To Do:" lists are not parallelisable until you have had tea/coffee as applicable.

That leads to stalling when the first item on the list is "Obtain Tea/Coffee". Your sister of course realised that tea was incoming without any effort on her part. However, "Update internal To Do: list" was second on the list, and so she couldn't break free until the tea actually arrived.

Once you've had the first tea of the day, you can parallelise as necessary, and thus circumvent similar stalls for the rest of the day.

I am laughing because that is exactly how my brain works, and it breaks down in the same way at 4pm when my research group goes to tea.