I've got a soft spot for lines that make you smile and roll your eyes at the same time, the kind you tuck into a sticky note for your brother's lunchbox. One of my favorites to quote is, "A brother is a friend given by nature." — it’s simple, a little classic, and it works for birthdays, graduations, or that random Tuesday when he helped you fix your bike. Another one I use when we’re being sappy is, "Brothers may drive you crazy, get into your stuff, and irritate you. However, if anyone else dares say so, a brother will defend you to the death." — anonymous, but painfully true in my house.
When I write a message I like to pair a quote with a tiny memory: "Happy Birthday — remember when we camped in the backyard and you ate all the S'mores? 'Brothers are playmates in the beginning and best friends for life.'" It’s personal and quotable. For short texts, I’ll toss in, "Nobody will ever understand me like you do," or go comic with, "Sometimes being a brother is even better than being a superhero." Mix the quote with something only you two share and it becomes gold rather than a line off a Hallmark card.