Dear [Name of Sister-in-law],
Happy 50th marriage anniversary to one of the best persons I have ever met in my whole life. My dear sister-in-law, you are one of the best gifts my brother could ever give me. Congratulations on this big day. It is really a great day to celebrate your love and relationship. Fifty long years!! It could not have been easy to deal but you people made it great to live. Today is your day to be in spotlight and celebrate love, joys, sorrows and happiness that you have accomplished together. You mean the whole world to me my darling sister. I admire you each growing day how you can be so perfect in your ways and actions.
I can’t believe it has been fifty long years of this wonderful union and I always feel the luckiest with you. Marrying you was the smartest thing my brother did. Jokes apart, I am so grateful to both of you to provide a direction to my life and supporting me during good and bad times. You are so special in your own way and you inspire me everyday to be a better me. You and brother are like salt and pepper, so incomplete without each other and so complete when you are together. Today is the day to look back with pride upon the fifty cherished years that you have spent side by side.
May your love grow stronger with each passing day. Congratulations.