My/Our (often implies family or close circle). Otouto (弟): Younger brother. Maji de (マジで): Seriously/Really. Dekainn (デカイン/デカい): Big/Huge/Large.
Aa… uchi no otouto maji de dekainn. “Ah… it’s just that my little brother is seriously huge.” uchi no otouto maji de dekainn