Hi, I need your help.
I wanna write my best friend a birthday caption in an Instagram story, but I'm not good with words. so I want you to correct me if there is anything wrong in the following sentence..

"Happiest of birthdays to the closest one to my heart, my twin sister, and the one who never fails to make me laugh with no extent. Love you endlessly."

Question

Hi, I need your help. 
I wanna write my best friend a birthday caption in an Instagram story, but I'm not good with words. so I want you to correct me if there is anything wrong in the following sentence..

"Happiest of birthdays to the closest one to my heart, my twin sister, and the one who never fails to make me laugh with no extent. Love you endlessly."

Answer

it seems great to me! :)

Answer

It's fine. Don't change anything. Particularly in a greeting card, "it's the thought that counts"--90% of the message is not in the words, just the fact that you bothered to write.

When you do want to write, especially to a close friend, you want it to "sound like you."

In an old song, "My Funny Valentine," the speaker says 
"Don't change a hair for me
Not if you care for me."

Don't change a word of it.

I will point out one small error but I don't think you should correct it. "With no extent" should be "with no limit."

Answer

It's beautiful don't change it. It sounds very special.