Offensive for a Birthday?

then np nothing wrong with a joke between siblings

 

No, I think it's really cute️️ But maybe ask your parents, just in case, ya know. And you know your sister best, so if you think she can take it then she probably can

 

Not offensive but if your sister is like that this letter suites here

 

I always tell my older sister 'Thank you, I try' when she calls me annoying. If she's like that then I don't think she would. But maybe add a winky face at the end or something so the letter seems more joking.