What words to write on your sister's birthday cake depends on your relationship with your sister and your wishes. Here are some possible options:

1. Simple blessing: Write "Happy birthday, sister!" " Or "Happy birthday to my sister!" This simple greeting expresses your birthday wishes to your sister.

2. Close blessing: Write "Sister, you are the most important person in my life. Happy birthday!" Or "Happy birthday to my dearest sister!" This kind of greeting expresses your deep feelings and blessings for your sister.

3. Personalized blessing: Write corresponding greetings according to your sister's hobbies or characteristics. For example, if my sister likes traveling, you can write "Happy birthday to my sister and have a nice trip!" Or if my sister likes music, you can write "Happy birthday to my sister, music will accompany you all your life!" "

4. meaningful wishes: write some meaningful wishes, such as "Happy birthday to my sister, may your life be full of love and laughter!" Or "I wish my sister a happy life and a happy birthday!" This kind of greeting expresses your blessing and good wishes to your sister.

No matter which greeting you choose, it should be sincere and heartfelt, expressing your birthday wishes and best wishes to your sister.