I would go with a bouquet with a prominent water lily. My sister is Cancer and I’m sure she would love it.
Here some other ideas:
The flower paired with Cancer is the ever-popular rose, sometimes called the queen of the garden. They will enjoy eclectic collection of flowers, trees and spices. Other flowers could be: Bear’s breeches, morning glory, geranium, lily, waterlily, cabbage, lotus, verbena, Southern magnolia, cow parsley, white flowers.