Children of the Lens by E.E. Smith

Karen Kinnison was worried. She, who had always been so steady, so sure of herself, had for weeks been conscious of a gradually increasing. what was it, anyway? Not exactly a loss of control. a _change_. a something that manifested itself in increasingly numerous fits of senseless -- sheerly idiotic -- stubbornness. And always and only it was directed at -- of all the people in the universe -- her brother. She got along with her sisters perfectly; their tiny tiffs barely rippled the surface of any of their minds. But any time her path of action crossed Kit's, it seemed, the profoundest depths of her being flared into opposition like exploding duodec. Worse than senseless and idiotic, it was inexplicable, for the feeling which the Five had for each other was much deeper than that felt by ordinary brothers and sisters.

She didn't want to fight with Kit. She _liked_ him! She liked to feel his mind _en rapport_ with hers, just as she liked to dance with him; their bodies as completely in accord as were their minds. No change of step or motion, however suddenly conceived and executed or however bizarre, had ever succeeded in taking the other by surprise or in marring by a millimeter the effortless precision of their performance. She could do things with Kit that would tie any other man into knots and break half of his bones. All other men were lumps. Kit was so far ahead of any other man in existence that there was simply no comparison. If she were Kit she would give her a going-over that would. or could even he --