One example of a quote about sisterhood that is suitable for a birthday message is "Sisters like you are diamonds. They sparkle, they are priceless, and they are truly a woman’s best friend. Happy birthday, sis." This quote is appropriate for adding a personal touch to cards and emails.
"I smile because you are my sister, I laugh because there is nothing you can do about it!" This is another example of a great quote for a sister's birthday card. Many inspirational quotes about sisters also have an element of humor. They help show a sister that she is loved while making her smile.