It's Drow tradition. Oldest daughter raises the next daughter when she is old enough an so on down the line. So Snadhya raised Sarv'swati who raised Sillice who raised Nishi'kanta who raised Zala'ess. If I remember properly it's supposed to help them both contribute to the clan by freeing up their mothers to do other things as well as give them practical experience before they have their own children.