Daisies
Daisies are a symbol of life and fertility that grows around the dead sister. The poet asks someone to tread slowly as "she can hear", almost as if she is alive.
Lily
The lily can be a symbol of her purity and innocence. The poet mentions that she was unaware as she grew into a woman. There is a poetic tendency to glorify the dead, and that seems to be the case in this poem too.
Golden
Golden again symbolizes purity of the sister's soul, which has now been tarnished in death.
White
White has dual connotations. It may symbolize the purity and innocence of the sister or the peace and death which now surrounds her. It symbolizes purity in the sense that the poet talks about the premature death of his sister, and it won't be wrong to assume that she was at an age of innocence when she died.
Rust
Rust is a paradoxical symbol. It symbolizes the life around the sister and also symbolizes decay. But the central meaning of rust seems to be the idea of decay. One of the central ideas explored by the poet is that of decay, and the way someone as beautiful and lively as his sister could just end up under the earth.
