Little sister has come to hang around you, Simon. Someone didn't have to be a Reader to feel the disapproval bubbling around him, so she's taken up a seat nearby. Out of the way so he can work, but near enough that they can have a conversation and she can intervene if any of his temporary patients gets grouchy with him. (He had beside manner, he does. It's just... particular.)

But the remark about him being of use...

"Synergy. The creation of a whole that is greater than the simple sum of its parts. From the Greek synergos, meaning 'working together'. You're not just Doctor Tam."