When sending birthday wishes to a sister, a good phrases to use is, "You are everything I could ever hope for in a sister and more. Happy birthday!" Another example of a good birthday phrase is, "Best wishes to the only girl I have ever dressed in princess clothes with!"
Some people prefer a birthday wish that is sentimental. For example, "There's no other friend like a sister, and no other sister is like you." For one sister to another, "There's a special kind of bond that only sisters enjoy and that the years can never break. Happy Birthday!" is a nice option. Be sure to express a heartfelt message without cliché.
Comical phrases are also good choices when sending wishes to a sister. For instance, a humorous birthday greeting could be, "Having a sister like you is like eating a box of chocolates--sweet, generous, and undeniably amazing!" Another funny phrase is, "Wishing you the happiest of birthdays and sending advanced apologies for the Christmas wrapping."
For a person sending wishes for a belated birthday, a good phrase is, "A snail once told me that it's never too late to wish a sister a happy birthday. I hope your day was filled with happiness and joy!"