The Oriel Window by Mary Louisa Molesworth

Thanks to the extra sleep which had come to Ferdy after all, he had not long to wait for Chrissie once he had wakened up "for good. " She was not allowed to see him till he had had his breakfast, for it was very important to keep up his strength with nourishing food, and "if you begin talking together, you know," said mamma, "Ferdy would get interested and excited, and very likely not feel inclined to eat anything. That is even the way sometimes when you are both quite well."

She was speaking to Chrissie about how careful she must be, if she were to be trusted to be with her brother, not to seem sad or dull, and yet to be very quiet -- "quietly cheerful, dear," she went on, "and if Ferdy is at all cross or peevish, you must just not mind."