They need a tragedy to bring them close first. Let’s say Ozai abandons Azula and she’s left with nothing, trying to survive in the Earth Kingdom, believing that nobody ever loved her and never will. Zuko finds her and they both try to escape the Fire Nation’s army, bonding in the process.
Azula believes that Zuko hates her!
Zuko is bewildered and sincerely denies it!
Azula and Zuko get closer!
Azula now trusts Zuko and vice versa!
Azula suddenly develops a crush on Zuko!
Azula accidentally kisses her brother when she’s caught up in a moment of celebration of something!
Zuko is conflicted! Azula is embarrassed of herself!
Etc. etc. would make a good story. In fact, it is. Check out a fanfic titled Abandoned Sparks, no kisses yet though. At least not intentional.