Sis is feeling much better this evening. I'm sure it has a lot to do with rest, but perhaps even more to do with her brother.
They have missed each other these last two days that Sis was home sick (though they aren't even in the same class anymore, only seeing each other on the bus and in passing.) Yesterday, she made him apple pie to welcome him home after school! For his part, he opted to stay at our house with the babysitter instead of going to the babysitter's house, so that Sis, whom I said had to stay here, could play, too. Today, Sis bought candy for him at the store and hid it for him to find. Now they're downstairs with their heads together playing Minion Rush or something. So sweet. I hardly feel like enforcing screen time limits with such camaraderie and so they are well over their daily limit.
Witnessing (and at times fostering) their close relationship has been one of the greatest joys of being their mom.