twins
孖 centers on the idea of twins: two identical children side by side, representing a pair born together.
孖 is formed by doubling the child component 子, directly representing two children together, i.e., twins. The exact historical development is uncertain, but the visual doubling is a straightforward semantic construction.
Two 子 (child) characters sit side by side like a pair of twins. Imagine identical twin children standing together.
For シ, picture the twins as two identical 'she' children: she -> シ, and the twins are two little girls.