The language of flowers became very popular in Victorian England, but the practice of assigning symbolic meaning to flowers, leaves and herbs has been going on for much longer than that. If you're giving flowers to your sister, make it a bouquet rich with meaning. Include flowers that convey the message you want to send; add a card to explain the symbolism of your flowers in case your sister doesn't speak the language as well as you do.
Daffodils symbolize fondness, respect and the feeling that the sun shines the brightest when you're with the recipient. They also convey faith, forgiveness and forthrightness -- all good messages from one sibling to another.
The word pansy comes from the French word pensee, which means to think, because of the flowers' resemblance to little faces nodding in deep thought. They symbolize remembrance of shared times, as well as merriment and "you are in my thoughts."
If you're sending a thank-you bouquet, include camellias and Canterbury bells, because they signify gratitude. Sweet peas are even more specific, symbolizing "thank you for a lovely time." Pink carnations also mean thank you.
Poppies mean consolation, while peonies convey healing. Pink tulips say "I care," while white tulips mean "I forgive you." Cosmos encourage her to find peace and asters, contentment. Coreopsis says, "Cheer up."
Just for fun, exchange flowers with your sister that harken to those days of childhood bickering. Geraniums can mean "you are a real friend," true, but they can also mean "you are stupid." Narcissus says "you're too vain," and yellow hyacinth symbolizes jealousy. Petunias can mean anger. Rhododendrons mean "you'd better watch out," and cyclamen says goodbye. Add a sprig of bells of Ireland, which signifies whimsy, so she will know you're just kidding.