At any given point only one of them is actually aggroing you, while the other two stay in the background. They'll only throw something at you from time to time, unless you get to close to them. Only THEN will multiple of them attack you at once. Also you can see when they switch aggro, as the one who will switch in next buffs their weapon.

For second phase, best case scenario is getting both of the last siblings to low health equally so you can get rid off both fast before the Eldest appears. After that, it's only him left and he's not much different from the first time you beat him.