Use a simple fill‑in‑the‑blank formula.
Write:
“Happy birthday to my _____ brother. Thanks for _____.”
Then plug in one real trait and one real thing he does.
Examples:
“funniest / always making me laugh”
“bravest / standing up for me”
“smartest / helping me through tough stuff”
Now keep it under 75 characters:
“Happy birthday to my funniest brother. Thanks for the laughs.”
“Happy birthday to my bravest brother. Thanks for backing me up.”