You might try bluemountain. They have a section in their online cards that feature cards to sisters. Many of them include very beautiful poems. Sometimes though the most heart felt one is one you've taken some time to write about your own sisterhood. One of my most cherished poems is one i wrote to my sisters during a time when i was struggling with to put it mildly - a sever case of homesickness...
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Beta Chapter Alumna and Founder
Alpha Sigma Kappa - Women in Technical Studies
Sisters by nature; Friends by Choice
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