You are now in your first full on fight. You have some optional objectives as well here. You need to Parry two attacks and Dodge two attacks. Both abilities are crucial in the many battles you'll find yourself in throughout the game.
Hugo, the first brother, will attack you. You will be prompted to parry his attacks. Just attack him rapidly after the first parry and you can push the advantage to the point he will not be able to retaliate against you.
The second brother, Victor, will attack you after you floor the first. Against him you will be prompted to dodge. Just do this twice as he attacks Arno to complete the optional objective. Then, parry him and push the advantage once again. Just after that, expect to be fighting both brothers at once. Take your time to defend yourself with dodging and parrying to keep an advantage through. Defeat them both to finish the fight. Arno will have to make a hasty exit after that.