"Y-Yes."

Is she showing a dissatisfied expression? Yumi hurriedly stood up and turned back as she replied. "Say it if you have any problems." If Sachiko-sama told her so, Yumi had no words to reply. If something could be done when she said so, she would say it. If doing anything would not move Sachiko-sama, then anything said would be just meaningless.

However Sachiko-sama didn't say as Yumi expected. Instead, a phrase without a subject reached Yumi's ears.

"Coming?"
"Pardon?"

Coming? It is the present continuous tense of the verb "come."
Oh, it's after the tests, so forget about it.
The problem now is: "who" is coming to "where" ?

What if the omitted subject is the first person pronoun "I," meaning Sachiko-sama herself? In that case the destination would the most appropriately be "here" or "school." You know, it is not common to say "I am coming to Paris." "Here" is most commonly used to specify the current location. Here is not Paris, so it should be "going to Paris."

But what is the point of saying "I am coming to school (here)" ?

Sachiko-sama's expression is in the tone of a question. Then, the subject should actually be "you" or "Yumi." Then to where is Yumi supposed to be "coming" ? Using the verb "coming" means the destination must be somewhere Sachiko-sama would be at--

Seeing the question marks flying out of Yumi's head, Sachiko-sama changed her wordings for one more time in amusement.

"W, Wha, Wha, Whaaaat?"
Upon hearing that, Yumi's heart nearly fell out.
Unable to decide where to place her startlement, Yumi lined up her palms facing out without any thought, and gave them a semi-circular swipe at shoulder height like a car's wiper.

Onee-sama's words were far beyond Yumi's wildest imaginations, that's why.
Exactly.

Why not come visit our summerhouse? Sachiko-sama had said.