ONCE AGAIN Patricia Ferguson has set a novel in Cornwall. Many of the characters from her last book, The Midwife’s Daughter, reappear here, along with plenty of new ones, as Ferguson continues to trace the changing lives of women between the two world wars.At the centre of Aren’t We Sisters? are Lettie, a devoted follower of Marie Stopes, sure that she will save the world through family planning; Norah, whose family were once among the aristocracy of the Cornish town of Silkhampton, but who is now struggling to make a viable life for herself; and Rae, a beautiful actress hiding away to await the birth of the baby she should not be having.   Lettie is in Cornwall to retrace something of her own family history, and she is cannily supplementing her income by arranging,