Zayn and Niall were half brothers aged 4 and 5. Months after Zayn was adopted into the Horan family they all got into a serious car accident
Maura, Bobby and Greg didn't make it out. But the two small boys did, and after healing, Zayn went back to the orphanage but with Niall as well.
Each boy was different, entirely and despite everyone trying to take home one or the other, neither boy left without the other
They made a promise to never leave each other, and they kept it as long as they could