The Beauty is also known as Caroline Crewler, Sophy's oldest sister. She is much admired by the rest of the family for being, you know, beautiful. But she is also unlucky in love: her husband is a worthless waste of space who leaves her widowed, with a baby. The Beauty comes to live with Traddles and Sophy by the end of the novel.
Sarah is one of Sophy Crewler's sisters. She has something wrong with her spine, which means that she has to stay lying down for at least a year. When David meets up with Traddles again in London when they are both starting out in their respective law offices, Traddles explains that one reason he can't marry Sophy quite yet is because she has to nurse Sarah.
Once Traddles and Sophy are happily married, these sisters come to stay with Traddles in his cramped little apartment while they are on break from school. But Traddles is so good-hearted that he takes pride in their intelligence and prettiness, as though they were his own daughters. Traddles is really a saint!