Louise might not be musical, but her sister, Caroline, definitely is. Her singing and piano playing aren't the only things that set Caroline apart from her sister, but they're probably the most notable. In fact, Caroline is so good that even Louise, who dreams of murdering her, can't hate on her singing chops:
Mr. Rice's hands went down, and from the center of the back row Caroline's voice came suddenly like a single beam of light across the darkness […]
It was a lonely, lonely sound, but so clear, so beautiful that I tightened my arms against my sides to keep from shaking, perhaps shattering. Then we were all singing, better than we had all night, better than we ever had, suddenly judged, damned, and purged in Caroline's light. She sang once more by herself, repeating the words of the first verse so quietly that I knew surely I would shatter when she went up effortlessly, sweetly, and oh, so softly, to the high G, holding it just a few seconds longer than humanly possible and then returning to the last few notes and to silence. (3.38, 40)
As much as Louise resents her sister, she's proud of her musical abilities. Music is the one thing that connects them. Yes, it's a tenuous connection, but still, it's a connection.
That's why there's something really special about the moment when Louise realizes her father is a singer, too. When he sings to the oysters, she's as moved by his love and expression of music as she is by Caroline's. The fact that her father isn't showy about his singing—unlike Caroline—makes it way cooler to Louise. After all, Caroline doesn't know her father has the voice of an angel; only Louise does.
It's pretty fitting that when Louise thinks of her sister in the last pages of the novel, she remembers her singing. Caroline may be vain and patronizing and showy, but the girl sure can sing. And maybe that's all the connection the two sisters need.