The fairest lady know I; so fair
She calls the mirror first cousin and dearest
She has many diamonds, though she is not rich
(She uses them in her work)
She always leaves her cloak upon the rack
She never cries, though sure she has good reason
Winter.
"Fair" in the sense of being pale (white as snow).
Ice and snow - first cousin to water, which reflects.
Many diamonds - I'd assume this refers to ice.
Cloak - probably snow.
Never cries - never rains, perhaps?
Has reason to cry - because of the harsh cold and dead trees, perhaps?
Second guess: Snow Maiden? In Russian folklore, she's the assistant to Grandfather Frost (so she's like Santa Claus's right-hand man, er, maiden).