It could indeed be Arthur Ransome, but in that book Dick’s telescope is more of a device for reading semaphore signals sent by another group of children. There is the stars book owned by Dick, and a few constellations are mentioned at the start. But the book is more about ice skating on the frozen lake. But yes, a brother and sister……. I find the question intriguing and at first I thought I knew the answer. But it was not so simple. On my shelf I still have an early 60s book from my childhood, Timothy’s Book of Space, but herein Timothy is the only one to receive instruction from his Dad. Although I cannot solve the riddle, instruction in the form of a series of long conversations between brother and sister is the theme of another book in my library by James Ferguson, An easy introduction to astronomy, for young gentlemen and ladies…. London MDCCLXXII. I will keep looking, but one certainly turns up interesting things along the way.